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Versions: (RFC 1519) 00 01 02 03 04 RFC 4632
GROW V. Fuller
Internet-Draft T. Li
Expires: December 12, 2005 Cisco Systems
June 10, 2005
Classless Inter-Domain Routing (CIDR): The Internet Address Assignment
and Aggregation Plan
draft-ietf-grow-rfc1519bis-02
Status of this Memo
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This Internet-Draft will expire on December 12, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo discusses the strategy for address assignment of the
existing 32-bit IPv4 address space with a view toward conserving the
address space and limiting the growth rate of global routing state.
This document obsoletes the original CIDR spec [RFC1519], with
changes made both to clarify the concepts it introduced and, after
more than twelve years, to update the Internet community on the
results of deploying the technology described.
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Table of Contents
1. History and Problem Description . . . . . . . . . . . . . . 3
1.1 Status updates to CIDR documents . . . . . . . . . . . . . 4
2. Classless addressing as a solution . . . . . . . . . . . . . 6
2.1 Basic concept and prefix notation . . . . . . . . . . . . 6
3. Address assignment and routing aggregation . . . . . . . . . 9
3.1 Aggregation efficiency and limitations . . . . . . . . . . 9
3.2 Distributed assignment of address space . . . . . . . . . 10
4. Routing implementation considerations . . . . . . . . . . . 11
4.1 Rules for route advertisement . . . . . . . . . . . . . . 12
4.2 How the rules work . . . . . . . . . . . . . . . . . . . . 13
4.3 A note on prefix filter formats . . . . . . . . . . . . . 13
4.4 Responsibility for and configuration of aggregation . . . 14
4.5 Route propagation and routing protocol considerations . . 15
5. Example of new address assignments and routing . . . . . . . 16
5.1 Address delegation . . . . . . . . . . . . . . . . . . . . 16
5.2 Routing advertisements . . . . . . . . . . . . . . . . . . 18
6. Domain Name Service considerations . . . . . . . . . . . . . 19
7. Transition to a long term solution . . . . . . . . . . . . . 21
8. Analysis of CIDR's effect on global routing state . . . . . 21
9. Conclusions and Recommendations . . . . . . . . . . . . . . 23
10. Security Considerations . . . . . . . . . . . . . . . . . . 23
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 25
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.1 Normative References . . . . . . . . . . . . . . . . . . 25
12.2 Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . 28
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1. History and Problem Description
What is now known as the Internet started as a research project in
the 1970s to design and develop a set of protocols that could be used
with many different network technologies to provide a seamless, end-
to-end facility for interconnecting a diverse set of end systems.
When determining how the 32-bit address space would be used, certain
assumptions were made about the number of organizations to be
connected, the number of end systems per organization, and total
number of end systems on the network. The end result was the
establishment (see [RFC791]) of three classes of networks: class A
(most significant address bits '00'), with 128 possible networks each
with 16777216 end systems (minus special bit values reserved for
network/broadcast addresses); class B (MSB '10'), with 16384 possible
networks each with 65536 end systems (less reserved values); and
class C (MSB '110'), with 2097152 possible networks each with 254 end
systems (256 bit combinations minus the reserved all-zeros and all-
ones patterns). The set of addresses with MSB '111' was reserved for
future use; parts of this were eventually defined (MSB '1110') for
use with IPv4 multicast and parts are still reserved as of the
writing of this document.
In the late 1980s, the expansion and commercialization of the former
research network resulted in the connection of many new organizations
to the rapidly-growing Internet and each new organization required an
address assignment according to the class A/B/C addressing plan. As
demand for new network numbers, particularly in the class B space
started to take on what appeared to be an exponential growth rate,
some members of the operations and engineering community started to
have concerns over the long-term scaling properties of the class
A/B/C system and began thinking about how to modify network number
assignment policy and routing protocols to better accommodate the
growth. In November, 1991, the IETF created the ROAD (Routing and
Addressing) group to examine the situation. This group met in
January, 1992 and identified three major problems:
1. Exhaustion of the class B network address space. One fundamental
cause of this problem is the lack of a network class of a size
which is appropriate for mid-sized organization; class C, with a
maximum of 254 host addresses, is too small, while class B, which
allows up to 65534 host addresses, is too large for most
organizations but was the best fit available for use with
subnetting.
2. Growth of routing tables in Internet routers beyond the ability
of current software, hardware, and people to effectively manage.
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3. Eventual exhaustion of the 32-bit IPv4 address space.
It was clear that then-current rates of Internet growth would cause
the first two problems to become critical some time between 1993 and
1995. Work already in progress on topological assignment of
addressing for CLNS, which was presented to the community at the
Boulder IETF in December of 1990, led to thoughts on how to re-
structure the 32-bit IPv4 address space to increase its lifespan.
Work in the ROAD group followed and eventually resulted in the
publication of [RFC1338] and later [RFC1519].
The design and deployment of CIDR was intended to solve these
problems by providing a mechanism to slow the growth of global
routing tables and to reduce the rate of consumption of IPv4 address
space. It did not and does not attempt to solve the third problem,
which is of a more long-term nature, but instead endeavors to ease
enough of the short to mid-term difficulties to allow the Internet to
continue to function efficiently while progress is made on a longer-
term solution.
More historical background on this effort and on the ROAD group may
be found in [RFC1380] and at [LWRD].
1.1 Status updates to CIDR documents
This memo renders obsolete and requests re-classification as Historic
the following RFCs describing CIDR usage and deployment:
o RFC 1467: Status of CIDR Deployment in the Internet
This Informational RFC described the status of CIDR deployment in
1993. As of 2005, CIDR has been thoroughly deployed, so this
status note only provides a historical data point.
o RFC 1481: IAB Recommendation for an Intermediate Strategy to
Address the Issue of Scaling
This very short Informational RFC described the IAB's endorsement
of the use of CIDR to address scaling issues. Because the goal of
RFC 1481 has been achieved, it is now only of historical value.
o RFC 1482: Aggregation Support in the NSFNET Policy-Based Routing
Database
This Informational RFC describes plans for support of route
aggregation, as specified by CIDR, on the NSFNET. Because the
NSFNET has long since ceased to exist and CIDR has been been
ubiquitously deployed, RFC 1482 now only has historical relevance.
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o RFC 1517: Applicability Statement for the Implementation of
Classless Inter-Domain Routing (CIDR)
This Standards Track RFC described where CIDR was expected to be
required and where it was expected to be (strongly) recommended.
With the full deployment of CIDR on the Internet, situations where
CIDR is not required are of only historical interest.
o RFC 1520: Exchanging Routing Information Across Provider
Boundaries in the CIDR Environment
This Informational RFC described transition scenarios where CIDR
was not fully supported for exchanging route information between
providers. With the full deployment of CIDR on the Internet, such
scenarios are no longer operationally relevant.
o RFC 1817: CIDR and Classful Routing
This Informational RFC described the implications of CIDR
deployment in 1995; it notes that formerly-classful addresses were
to be allocated using CIDR mechanisms and describes the use of a
default route for non-CIDR-aware sites. With the full deployment
of CIDR on the Internet, such scenarios are no longer
operationally relevant.
o RFC 1878: Variable Length Subnet Table For IPv4
This Informational RFC provided a table of pre-calculated subnet
masks and address counts for each subnet size. With the
incorporation of a similar table into this document (see
Section 2.1), it is no longer necessary to document it in a
separate RFC.
o RFC 2036: Observations on the use of Components of the Class A
Address Space within the Internet
This Informational RFC described several operational issues
associated with the allocation of classless prefixes from
previously-classful address space. With the full deployment of
CIDR on the Internet and more than half a dozen years of
experience making classless prefix allocations out of historical
"class A" address space, this RFC now has only historical value.
o RFC 1518: An Architecture for IP Address Allocation with CIDR
This Standards Track RFC discussed routing and address aggregation
considerations at some length. Some of these issues are
summarized in this document in section Section 2.1. Because
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address assignment policies and procedures now reside mainly with
the RIRs, it is not appropriate to try to document those practices
in a Standards Track RFC. In addition, [RFC3221] also describes
many of the same issues from point of view of the routing system.
2. Classless addressing as a solution
The solution that the community created was to deprecate the Class
A/B/C network address assignment system in favor of using
"classless", hierarchical blocks of IP addresses (referred to as
prefixes). The assignment of prefixes is intended to roughly follow
the underlying Internet topology so that aggregation can be used to
facilitate scaling of the global routing system. One implication of
this strategy is that prefix assignment and aggregation is generally
done according to provider-subscriber relationships, since that is
how the Internet topology is determined.
When originally proposed in [RFC1338] and [RFC1519], this addressing
plan was intended to be a relatively short-term response, lasting
approximately three to five years during which a more permanent
addressing and routing architecture would be designed and
implemented. As can be inferred from the dates on the original
documents, CIDR has far outlasted its anticipated lifespan and has
become the mid-term solution to the problems described above.
Coupled with address management strategies implemented by the
Regional Internet Registries (see [NRO] for details), the deployment
of CIDR-style addressing has also reduced the rate at which IPv4
address space has been consumed, thus providing short-to-medium-term
relief to problem #3 described above.
Note that, as defined, this plan neither requires nor assumes the re-
assignment of those parts of the legacy "class C" space that are not
amenable to aggregation (sometimes called "the swamp"). Doing so
would somewhat reduce routing table sizes (current estimate is that
"the swamp" contains approximately 15,000 entries) though at a
significant renumbering cost. Similarly, there is no hard
requirement that any end site renumber when changing transit service
provider but end sites are encouraged do so to eliminate the need for
explicit advertisement of their prefixes into the global routing
system.
2.1 Basic concept and prefix notation
In the simplest sense, the change from Class A/B/C network numbers to
classless prefixes is to make explicit which bits in a 32-bit IPv4
address are interpreted as the network number (or prefix) associated
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with a site and which are the used to number individual end systems
within the site. In CIDR notation, a prefix is shown as a 4-octet
quantity, just like a traditional IPv4 address or network number,
followed by the "/" (slash) character, followed by a decimal value
between 0 and 32 that describes the number of significant bits.
For example, the legacy "class B" network 172.16.0.0, with an implied
network mask of 255.255.0.0, is defined as the prefix 172.16.0.0/16,
the "/16" indicating that the mask to extract the network portion of
the prefix is a 32-bit value where the most significant 16 bits are
ones and the least significant 16 bits are zeros. Similarly, the
legacy "class C" network number 192.168.99.0 is defined as the prefix
192.168.99.0/24 - the most significant 24 bits are ones and the least
significant 8 bits are zeros.
Using classless prefixes with explicit prefix lengths allows much
more flexible matching of address space blocks to actual need. Where
formerly only three network sizes were available, prefixes may be
defined to describe any power-of-two-sized block of between one and
2^32 end system addresses. In practice, the unallocated pool of
addresses is administered by the Internet Assigned Numbers Authority
([IANA]). The IANA makes allocations from this pool to Regional
Internet Registries, as required. These allocations are made in
contiguous bit-aligned blocks of 2^24 addresses (a.k.a. /8 prefixes).
The RIRs, in turn, allocate or assign smaller address blocks to Local
Internet Registries (LIRs) or Internet Service Providers (ISPs).
These entities may make direct use of the assignment (as would
commonly be the case for an ISP) or may make further sub-allocations
of addresses to their customers. These RIR address assignments vary
according to the needs of each ISP or LIR. For example, a large ISP
might be allocated an address block of 2^17 addresses (a /15 prefix)
while a smaller ISP may be allocated an address block of 2^11
addresses (a /21 prefix).
Note that the terms "allocate" and "assign" have specific meaning in
the Internet address registry system; "allocate" refers to the
delegation of a block of address space to an organization which is
expected to perform further sub-delegations while "assign" is used
for sites that directly use (i.e. number individual hosts) the block
of addresses received.
The following table provides a convenient short-cut to all of the
CIDR prefix sizes, showing the number of addresses possible in each
prefix and the number of prefixes of that size that may be numbered
in the 32-bit IPv4 address space:
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notation addrs/block # blocks
-------- ----------- ----------
n.n.n.n/32 1 4294967296 "host route"
n.n.n.x/31 2 2147483648 "[RFC3021] p2p link"
n.n.n.x/30 4 1073741824
n.n.n.x/29 8 536870912
n.n.n.x/28 16 268435456
n.n.n.x/27 32 134217728
n.n.n.x/26 64 67108864
n.n.n.x/25 128 33554432
n.n.n.0/24 256 16777216 legacy "class C"
n.n.x.0/23 512 8388608
n.n.x.0/22 1024 4194304
n.n.x.0/21 2048 2097152
n.n.x.0/20 4096 1048576
n.n.x.0/19 8192 524288
n.n.x.0/18 16384 262144
n.n.x.0/17 32768 131072
n.n.0.0/16 65536 65536 legacy "class B"
n.x.0.0/15 131072 32768
n.x.0.0/14 262144 16384
n.x.0.0/13 524288 8192
n.x.0.0/12 1048576 4096
n.x.0.0/11 2097152 2048
n.x.0.0/10 4194304 1024
n.x.0.0/9 8388608 512
n.0.0.0/8 16777216 256 legacy "class A"
x.0.0.0/7 33554432 128
x.0.0.0/6 67108864 64
x.0.0.0/5 134217728 32
x.0.0.0/4 268435456 16
x.0.0.0/3 536870912 8
x.0.0.0/2 1073741824 4
x.0.0.0/1 2147483648 2
0.0.0.0/0 4294967296 1 "default route"
n is an 8-bit, decimal octet value.
x is a 1 to 7 bit value, base on the prefix length, shifted into the
most significant bits of the octet and converted into decimal form;
the least significant bits of the octet are zero.
In practice, prefixes of length shorter than 8 are not allocated or
assigned though routes to such short prefixes may exist in routing
tables if or when aggressive aggregation is performed. As of the
writing of this document, no such routes are seen in the global
routing system but operator error and other events have caused some
of them (i.e. 128.0.0.0/1 and 192.0.0.0/2) to be observed in some
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networks at some times in the past.
3. Address assignment and routing aggregation
Classless addressing and routing was initially developed primarily to
improve the scaling properties of routing on the global Internet.
Because the scaling of routing is very tightly coupled to the way
that addresses are used, deployment of CIDR had implications for the
way in which addresses were assigned.
3.1 Aggregation efficiency and limitations
The only commonly-understood method for reducing routing state on a
packet-switched network is through aggregation of information. For
CIDR to succeed in reducing the size and growth rate of the global
routing system, the IPv4 address assignment process needed to be
changed to make possible the aggregation of routing information along
topological lines. Since, in general, the topology of the network is
determined by the service providers who have built it, topologically-
significant address assignments are necessarily service-provider
oriented.
Aggregation is simple for an end site which is connected to one
service provider: it uses address space assigned by its service
provider and that address space is a small piece of a larger block
allocated to the service provider. No explicit route is needed for
the end site - the service provider advertises a single aggregate
route for the larger block; this advertisement provides reachability
and routeability for all of the customers numbered in the block.
There are two, more complex, situations that reduce the effectiveness
of aggregation:
o An organization which is multi-homed. Because a multi-homed
organization must be advertised into the system by each of its
service providers, it is often not feasible to aggregate its
routing information into the address space of any one of those
providers. Note that the organization still may receive its
address assignment out of a service provider's address space
(which has other advantages), but a route to the organization's
prefix must still be explicitly advertised by all of its service
providers. For this reason, the global routing cost for a multi-
homed organization is generally the same as it was prior to the
adoption of CIDR.
o An organization which changes service provider but does not
renumber. This has the effect of "punching a hole" in one of the
original service provider's aggregated route advertisements. CIDR
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handles this situation by requiring the newer service provider to
advertise a specific advertisement for the re-homed organization;
this advertisement is preferred over provider aggregates because
it is a longer match. To maintain efficiency of aggregation, it
is recommended that an organization which changes service
providers plan to eventually migrate its network into a an prefix
assigned from its new provider's address space. To this end, it
is recommended that mechanisms to facilitate such migration, such
as dynamic host address assignment using [RFC2131]) be deployed
wherever possible, and that additional protocol work be done to
develop improved technology for renumbering.
Note that some aggregation efficiency gain can still be had for
multi-homed sites (and, in general, for any site composed of
multiple, logical IPv4 networks) - by allocating a contiguous power-
of-two block address space to the site (as opposed to multiple,
independent prefixes) the site's routing information may be
aggregated into a single prefix. Also, since the routing cost
associated with assigning a multi-homed site out of a service
provider's address space is no greater than the old method of
sequential number assignment by a central authority, it makes sense
to assign all end-site address space out of blocks allocated to
service providers.
It is also worthwhile to mention that since aggregation may occur at
multiple levels in the system, it may still be possible to aggregate
these anomalous routes at higher levels of whatever hierarchy may be
present. For example, if a site is multi-homed to two relatively
small providers that both obtain connectivity and address space from
the same large provider, then aggregation by the large provider of
routes from the smaller networks will include all routes to the
multi-homed site. The feasibility of this sort of second-level
aggregation depends on whether topological hierarchy exists between a
site, its directly-connected providers, and other providers to which
they are connected; it may be practical in some regions of the global
Internet but not in others.
Note: in the discussion and examples which follow, prefix notation is
used to represent routing destinations. This is used for
illustration only and does not require that routing protocols use
this representation in their updates.
3.2 Distributed assignment of address space
In the early days of the Internet, IPv4 address space assignment was
performed by the central Network Information Center (NIC). Class
A/B/C network numbers were assigned in essentially arbitrary order,
roughly according to the size of the organizations that requested
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them. All assignments were recorded centrally and no attempt was
made to assign network numbers in a manner that would allow routing
aggregation.
When CIDR was originally deployed, the central assignment authority
continued to exist but changed its procedures to assign large blocks
of "Class C" network numbers to each service provider. Each service
provider, in turn, assigned bitmask-oriented subsets of the
provider's address space to each customer. This worked reasonably
well as long as the number of service providers was relatively small
and relatively constant but did not scale well as the number of
service providers grew at a rapid rate.
As the Internet started to expand rapidly in the 1990s, it became
clear that a single, centralized address assignment authority was
problematic. This function began being de-centralized when address
space assignment for European Internet sites was delegated in bit-
aligned blocks of 16777216 addresses (what CIDR would later define as
a /8) to the RIPE NCC ([RIPE]), effectively making it the first of
the RIRs. Since then, address assignment has been formally
distributed as a hierarchical function with IANA, the RIRs, and the
service providers. Removing the bottleneck of a single organization
having responsibility for the global Internet address space greatly
improved the efficiency and response time for new assignments.
Hierarchical delegation of addresses in this manner implies that
sites with addresses assigned out of a given service provider are,
for routing purposes, part of that service provider and will be
routed via its infrastructure. This implies that routing information
about multi-homed organizations, i.e., organizations connected to
more than one network service provider, will still need to be known
by higher levels in the hierarchy.
A historical perspective on these issues is described in [RFC1518].
Additional discussion may also be found in [RFC3221].
4. Routing implementation considerations
With the change from classful network numbers to classless prefixes,
it is not possible to infer the network mask from the initial bit
pattern of an IPv4 address. This has implications for how routing
information is stored and propagated. Network masks or prefix
lengths must be explicitly carried in routing protocols. Interior
routing protocols such as OSPF [RFC2178], IS-IS [RFC1195], RIPv2
[RFC2453], and Cisco EIGRP, and the BGP4 exterior routing protocol
[RFC1771] all support this functionality, having been developed or
modified as part of the deployment of classless inter-domain routing
during the 1990s.
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Older interior routing protocols, such as RIP [RFC1058], HELLO, and
Cisco IGRP, and older exterior routing protocols, such as EGP
[RFC904], do not support explicit carriage of prefix length/mask and
thus cannot be effectively used on the Internet in other than very
limited, stub configurations. While their use may be appropriate in
simple, legacy end-site configurations, they are considered obsolete
and should NOT be used in transit networks connected to the global
Internet.
Similarly, routing and forwarding tables in layer-3 network equipment
must be organized to store both prefix and prefix length or mask.
Equipment which organizes its routing/forwarding information
according to legacy class A/B/C network/subnet conventions cannot be
expected to work correctly on networks connected to the global
Internet; use of such equipment is not recommended. Fortunately,
very little such equipment is in use today.
4.1 Rules for route advertisement
1. Routing to all destinations must be done on a longest-match basis
only. This implies that destinations which are multi-homed
relative to a routing domain must always be explicitly announced
into that routing domain - they cannot be summarized (this makes
intuitive sense - if a network is multi-homed, all of its paths
into a routing domain which is "higher" in the hierarchy of
networks must be known to the "higher" network).
2. A router which generates an aggregate route for multiple, more-
specific routes must discard packets which match the aggregate
route but not any of the more-specific routes. In other words,
the "next hop" for the aggregate route should be the null
destination. This is necessary to prevent forwarding loops when
some addresses covered by the aggregate are not reachable.
Note that during failures, partial routing of traffic to a site which
takes its address space from one service provider but which is
actually reachable only through another (i.e., the case of a site
which has changed service providers) may occur because such traffic
will be forwarded along the path advertised by the aggregated route.
Rule #2 will prevent packet mis-delivery by causing such traffic to
be discarded by the advertiser of the aggregated route, but the
output of "traceroute" and other similar tools will suggest that a
problem exists within that network rather than in the network which
is no longer advertising the more-specific prefix. This may be
confusing to those trying to diagnose connectivity problems; see the
example in Section 5.2 for details. A solution to this perceived
"problem" is beyond the scope of this document - it lies with better
education of the user/operator community, not in routing technology.
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An implementation following these rules should also be generalized,
so that an arbitrary network number and mask are accepted for all
routing destinations. The only outstanding constraint is that the
mask must be left contiguous. Note that the degenerate route to
prefix 0.0.0.0/0 is used as a default route and MUST be accepted by
all implementations. Further, to protect against accidental
advertisements of this route via the inter-domain protocol, this
route should only be advertised when a router is explicitly
configured to do so - never as a non-configured, "default" option.
4.2 How the rules work
Rule #1 guarantees that the routing algorithm used is consistent
across implementations and consistent with other routing protocols,
such as OSPF. Multi-homed networks are always explicitly advertised
by every service provider through which they are routed even if they
are a specific subset of one service provider's aggregate (if they
are not, they clearly must be explicitly advertised). It may seem as
if the "primary" service provider could advertise the multi-homed
site implicitly as part of its aggregate, but the assumption that
longest-match routing is always done causes this not to work.
Rule #2 guarantees that no routing loops form due to aggregation.
Consider a site that has been assigned 192.168.64/19 by its "parent"
provider that has 192.168.0.0/16. The "parent" network will
advertise 192.168.0.0/16 to the "child" network. If the "child"
network were to lose internal connectivity to 192.168.65.0/24 (which
is part of its aggregate), traffic from the "parent" to the to the
"child" destined for 192.168.65.1 will follow the "child's"
advertised route. When that traffic gets to the "child", however,
the mid-level *must not* follow the route 192.168.0.0/16 back up to
the "parent", since that would result in a forwarding loop. Rule #2
says that the "child" may not follow a less-specific route for a
destination which matches one of its own aggregated routes
(typically, this is implemented by installing a "discard" or "null"
route for all aggregated prefixes which one network advertises to
another). Note that handling of the "default" route (0.0.0.0/0) is a
special case of this rule - a network must not follow the default to
destinations which are part of one of it's aggregated advertisements.
4.3 A note on prefix filter formats
Systems which process route announcements must be able to verify that
information which they receive is acceptable according to policy
rules. Implementations which filter route advertisements must allow
masks or prefix lengths in filter elements. Thus, filter elements
which formerly were specified as:
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accept 172.16.0.0
accept 172.25.120.0.0
accept 172.31.0.0
deny 10.2.0.0
accept 10.0.0.0
now look something like:
accept 172.16.0.0/16
accept 172.25.0.0/16
accept 172.31.0.0/16
deny 10.2.0.0/16
accept 10.0.0.0/8
This is merely making explicit the network mask which was implied by
the class A/B/C classification of network numbers. It is also useful
to enhance filtering capability to allow the match of a prefix and
all more-specific prefixes with the same bit pattern; fortunately,
this functionality has been implemented by most vendors of equipment
used on the Internet.
4.4 Responsibility for and configuration of aggregation
Under normal circumstances, a routing domain (or "Autonomous System")
which has been allocated or assigned a set of prefixes has sole
responsibility for aggregation of those prefixes. In the usual case,
the AS will install configuration in one or more of its routers to
generate aggregate routes based on more-specific routes known to its
internal routing system; these aggregate routes are advertised into
the global routing system by the border routers for the routing
domain. The more-specific internal routes which overlap with the
aggregate routes should not be advertised globally. In some cases,
an AS may wish to delegate aggregation responsibility to another AS
(for example, a customer may wish for its service provider to
generate aggregated routing information on its behalf); in such
cases, aggregation is performed by a router in the second AS based on
the routes that it receives from the first combined with configured
policy information describing how those routes should be aggregated.
It should be mentioned that one provider may choose to perform
aggregation on the routes it receives from another without explicit
agreement; this is termed "proxy aggregation". This can be a useful
tool for reducing the amount of routing state that an AS must carry
and propagate to its customers and neighbors, proxy aggregation can
also create inconsistencies in global routing state. Consider what
happens if both AS 2 and 3 receive routes from AS 1 but AS 2 performs
proxy aggregation while AS 3 does not. Other AS's which receive
transit routing information from both AS 2 and AS 3 will see an
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inconsistent view of the routing information originated by AS 1.
This may cause an unexpected shift of traffic toward AS 1 through AS
3 for AS 3's customers and any others receiving transit routes from
AS 3. Because proxy aggregation can cause unanticipated consequences
for parts of the Internet that have no relationship with either the
source of the aggregated routes or the party providing aggregation,
it should be used with extreme caution.
Configuration of the routes to be combined into aggregates is an
implementation of routing policy and does require some manually-
maintained information. As an addition to the information that must
be maintained for a set of routeable prefixes, aggregation
configuration is typically just a line or two defining the range of
the block of IPv4 addresses to aggregate. A site performing its own
aggregation is doing so for address blocks that it has been assigned;
a site performing aggregation on behalf of another knows this
information based on an agreement to delegate aggregation. Assuming
a best common practice for network administrators to exchange lists
of prefixes to accept from one and other, configuration of
aggregation information does not introduce significant additional
administrative overhead.
The generation of an aggregate route is usually specified either
statically or in response to learning an active dynamic route for a
prefix contained within the aggregate route. If such dynamic
aggregate route advertisement is done, care should be taken that
routes are not excessively added or withdrawn (known as "route
flapping"); in general, a dynamic aggregate route advertisement is
added when at least one component of the aggregate becomes reachable
and it is withdrawn only when all components become unreachable.
Properly configured, aggregated routes are more stable than non-
aggregated routes and thus improve global routing stability.
Implementation note: aggregation of the "Class D" (multicast) address
space is beyond the scope of this document.
4.5 Route propagation and routing protocol considerations
Prior to the original deployment of CIDR, common practice was to
propagate routes learned via exterior routing protocols (i.e. EGP or
BGP) through a site's interior routing protocol (typically, OSPF,
IS-IS, or RIP). This was done to ensure that consistent and correct
exit points were chosen for traffic destined to a destination learned
through those protocols. Four evolutionary effects -- the advent of
CIDR, explosive growth of global routing state, widespread adoption
of BGP4, and a requirement to propagate full path information -- have
combined to deprecate that practice. To ensure proper path
propagation and prevent inter-AS routing inconsistency (BGP4's loop
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detection/prevention mechanism requires full path propagation),
transit networks must use internal BGP (iBGP) for carrying routes
learned from other providers both within and through their networks.
5. Example of new address assignments and routing
5.1 Address delegation
Consider the block of 524288 (2^19) addresses beginning with
10.24.0.0 and ending with 10.31.255.255 allocated to a single network
provider, "PA". This is equivalent in size to a block of 2048 legacy
"class C" network numbers (or /24s). A classless route to this block
would be described as 10.24.0.0 with mask of 255.248.0.0 the prefix
10.24.0.0/13.
Assume this service provider connects six sites in the following
order (significant because it demonstrates how temporary "holes" may
form in the service provider's address space):
o "C1" requiring fewer than 2048 addresses (/21 or 8 x /24)
o "C2" requiring fewer than 4096 addresses (/20 or 16 x /24)
o "C3" requiring fewer than 1024 addresses (/22 or 4 x /24)
o "C4" requiring fewer than 1024 addresses (/22 or 4 x /24)
o "C5" requiring fewer than 512 addresses (/23 or 2 x /24)
o "C6" requiring fewer than 512 addresses (/23 or 2 x /24)
In all cases, the number of IPv4 addresses "required" by each site is
assumed to allow for significant growth. The service provider
delegates its address space as follows:
o C1: assign 10.24.0 through 10.24.7. This block of networks is
described by the route 10.24.0.0/21 (mask 255.255.248.0)
o C2: assign 10.24.16 through 10.24.31. This block is described by
the route 10.24.16.0/20 (mask 255.255.240.0)
o C3: assign 10.24.8 through 10.24.11. This block is described by
the route 10.24.8.0/22 (mask 255.255.252.0)
o C4: assign 10.24.12 through 10.24.15. This block is described by
the route 10.24.12.0/22 (mask 255.255.252.0)
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o C5: assign 10.24.32 and 10.24.33. This block is described by the
route 10.24.32.0/23 (mask 255.255.254.0)
o C6: assign 10.24.34 and 10.24.35. This block is described by the
route 10.24.34.0/23 (mask 255.255.254.0)
These six sites should be represented as six prefixes of varying size
within the provider IGP. If, for some reason, the provider were to
use an obsolete IGP that doesn't support classless routing or
variable-length subnets, then then explicit routes all /24s will have
to be carried.
To make this example more realistic, assume that C4 and C5 are multi-
homed through some other service provider, "PB". Further assume the
existence of a site "C7" which was originally connected to "RB" but
has moved to "PA". For this reason, it has a block of network
numbers which are assigned out "PB"'s block of (the next) 2048 x /24.
o C7: assign 10.32.0 through 10.32.15. This block is described by
the route 10.32.0.0/20 (mask 255.255.240.0)
For the multi-homed sites, assume that C4 is advertised as primary
via "RA" and secondary via "RB"; C5 is primary via "RB" and secondary
via "RA". In addition, assume that "RA" and "RB" are both connected
to the same transit service provider "BB".
Graphically, this topology looks something like this:
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10.24.0.0 -- 10.24.7.0__ __10.32.0.0 - 10.32.15.0
C1: 10.24.0.0/21 \ / C7: 10.32.0.0/20
\ /
+----+ +----+
10.24.16.0 - 10.24.31.0_ | | | |
C2: 10.24.16.0/20 \ | | _10.24.12.0 - 10.24.15.0__ | |
\| | / C4: 10.24.12.0/20 \ | |
| |/ \| |
10.24.8.0 - 10.24.11.0___/| PA |\ | PB |
C3: 10.24.8.0/22 | | \__10.24.32.0 - 10.24.33.0___| |
| | C5: 10.24.32.0/23 | |
| | | |
10.24.34.0 - 10.24.35.0__/| | | |
C6: 10.24.34.0/23 | | | |
+----+ +----+
|| ||
routing advertisements: || ||
|| ||
10.24.12.0/22 (C4) || 10.24.12.0/22 (C4) ||
10.32.0.0/20 (C7) || 10.24.32.0/23 (C5) ||
10.24.0.0/13 (PA) || 10.32.0.0/13 (PB) ||
|| ||
VV VV
+---------- BACKBONE NETWORK BB ----------+
5.2 Routing advertisements
To follow rule #1, PA will need to advertise the block of addresses
that it was given and C7. Since C4 is multi-homed and primary
through PA, it must also be advertised. C5 is multi-homed and
primary through PB. In principal (and in the example above), it need
not be advertised since longest match by PB will automatically select
PB as primary and the advertisement of PA's aggregate will be used as
a secondary. In actual practice, C5 will normally be advertised via
both providers.
Advertisements from "PA" to "BB" will be:
10.24.12.0/22 primary (advertises C4)
10.32.0.0/20 primary (advertises C7)
10.24.0.0/13 primary (advertises remainder of PA)
For PB, the advertisements must also include C4 and C5 as well as
it's block of addresses.
Advertisements from "PB" to "BB" will be:
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10.24.12.0/22 secondary (advertises C4)
10.24.32.0/23 primary (advertises C5)
10.32.0.0/13 primary (advertises remainder of RB)
To illustrate the problem diagnosis issue mentioned in Section 4.1,
consider what happens if PA loses connectivity to C7 (the site which
is assigned out of PB's space). In a stateful protocol, PA will
announce to BB that 10.32.0.0/20 has become unreachable. Now, when
BB flushes this information out of its routing table, any future
traffic sent through it for this destination will be forwarded to PB
(where it will be dropped according to Rule #2) by virtue of PB's
less specific match 10.32.0.0/13. While this does not cause an
operational problem (C7 is unreachable in any case), it does create
some extra traffic across "BB" (and may also prove confusing to
someone trying to debug the outage with "traceroute"). A mechanism
to cache such unreachable state might be nice but is beyond the scope
of this document.
6. Domain Name Service considerations
One aspect of Internet services which was notably affected by the
move to CIDR was the mechanism used for address-to-name translation:
the IN-ADDR.ARPA zone of the domain system. Because this zone is
delegated on octet boundaries only, the move to an address assignment
plan which uses bitmask-oriented addressing caused some increase in
work for those who maintain parts of the IN-ADDR.ARPA zone.
As described above, the IN-ADDR.ARPA zone is necessarily organized
along octet boundaries. Prior to the adoption of CIDR, IN-ADDR.ARPA
was also constrained such that delegations were only permitted along
legacy, class A/B/C network number boundaries. This created a
difficult situation for more flexible, CIDR prefixes. Consider a
hypothetical large network provider named "BigNet" which has been
allocated the block 10.4.0.0 through 10.7.255.0 (the CIDR prefix
10.4.0.0/14). Under the old delegation policies, the top-level IN-
ADDR.ARPA domain servers would need to have 1024 entries of the form:
0.4.10.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
1.4.10.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
....
255.7.10.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
By revising the policy as described above, this was reduced to four
delegation records:
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4.10.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
5.10.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
6.10.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
7.10.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
The provider then maintains further delegations of naming authority
for each individual /24 which it assigns, rather than having each
registered separately. Note that due to the way the DNS is designed,
it is still possible for the top-level IN-ADDR.ARPA name servers to
maintain the delegation information for individual networks for which
the provider is unwilling or unable to do so. The example above
illustrates only the records for a single name server. In the normal
case, there are usually several name servers for each domain, thus
the size of the examples will double or triple in the common cases.
For BIG.NET to assign a blocks smaller than /24 to its customers, it
can similarly delegate DNS authority for those addresses. For
example, if it were to assign 10.4.99.64/26 to its customer
CUSTONE.COM and 10.4.99.128/27 to its customer CUSTTWO.COM, it could
add the following records to delegate DNS for the addresses to those
customers:
64.99.4.10.IN-ADDR.ARPA. IN NS NS1.CUSTONE.COM.
65.99.4.10.IN-ADDR.ARPA. IN NS NS1.CUSTONE.COM.
....
127.99.4.10.IN-ADDR.ARPA. IN NS NS1.CUSTONE.COM
128.99.4.10.IN-ADDR.ARPA. IN NS NS1.CUSTTWO.COM
129.99.4.10.IN-ADDR.ARPA. IN NS NS1.CUSTTWO.COM
....
159.99.4.10.IN-ADDR.ARPA. IN NS NS1.CUSTTWO.COM
And if BIG.NET also assigned 10.4.99.160/30 to CUSTTHREE.COM but this
customer did not want to run its own DNS, BIG.NET could provide that
service for the customer by installing the appropriate PTR records:
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160.99.4.10.IN-ADDR.ARPA. IN PTR NET.CUSTTHREE.COM.
161.99.4.10.IN-ADDR.ARPA. IN PTR HOST1.CUSTTHREE.COM.
162.99.4.10.IN-ADDR.ARPA. IN PTR HOST2.CUSTTHREE.COM.
163.99.4.10.IN-ADDR.ARPA. IN PTR BCAST.CUSTTHREE.COM.
See [RFC2317] for a much more detailed discussion of DNS delegation
with classless addressing.
7. Transition to a long term solution
CIDR was designed to be a short-term solution to the problems of
routing state and address depletion on the IPv4 Internet. It does
not change the fundamental Internet routing or addressing
architectures. It is not expected to affect any plans for transition
to a more long-term solution except, perhaps, by delaying the urgency
of developing such a solution.
8. Analysis of CIDR's effect on global routing state
When CIDR was first proposed in the early 1990s, the original authors
made some observations about the growth rate of global routing state
and offered projections on how CIDR deployment would, hopefully,
reduce what appeared to be exponential growth to a more sustainable
rate. Since that deployment, an ongoing effort, called "The CIDR
Report" [CRPT] has attempted to quantify and track that growth rate.
What follows is a brief summary of the CIDR report as of March, 2005,
with an attempt to explain the various patterns of and change in
growth rate that have occurred since measurements of the size of
global routing state began in 1988.
Examining the graph of "Active BGP Table Entries" [CBGP] there appear
to be several different growth trends with distinct inflection points
reflecting changes in policy and practice. The trends and events
which are believed to have caused them were:
1. Exponential growth at the far left of the graph. This represents
the period of early expansion and commercialization of the former
research network, from the late 1980s through approximately 1994.
The major driver for this growth was a lack of aggregation
capability for transit providers, and the widespread use of
legacy Class C allocations for end sites. Each time a new site
was connected to the global Internet, one or more new routing
entries were generated.
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2. Acceleration of the exponential trend in late 1993 and early 1994
as CIDR supernet" blocks were first assigned by the NIC and
routed as separate legacy class-C networks by service provider.
3. A sharp drop in 1994 as BGP4 deployment by providers allowed
aggregation of the "supernet" blocks. Note that the periods of
largest declines in the number of routing table entries typically
correspond to the weeks following each meeting of the IETF CIDR
Deployment Working Group.
4. Roughly linear growth from mid-1994 to early 1999 as CIDR-based
address assignments were made and aggregated routes added
throughout the network.
5. A new period of exponential growth again from early 1999 until
2001 as the "high-tech bubble" fueled both rapid expansion of
Internet as well as a large increase in more-specific route
advertisements for multi-homing and traffic engineering.
6. Flattening of growth through 2001 caused by a combination of the
"dot-com bust", which caused many organizations to cease
operations, and the "CIDR police" [CPOL] work aimed at improving
aggregation efficiency.
7. Roughly linear growth through 2002 and 2003. This most likely
represents a resumption of the "normal" growth rate observed
before the "bubble" as well as an end to the "CIDR Police"
effort.
8. A more recent trend of exponential growth beginning in 2004. The
best explanation would seem to be an improvement of the global
economy driving increased expansion of the Internet and the
continued absence of the "CIDR Police" effort, which previously
served as an educational tool for new providers to improve
aggregation efficiency. There have also been some cases where
service providers have deliberately de-aggregated prefixes in an
attempt to mitigate security problems caused by conflicting route
advertisements (see Section 10). While this behavior may solve
the short-term problems seen by such providers, it is
fundamentally non-scalable and quite detrimental to the community
as a whole. In addition, there appear to be many providers
advertising both their allocated prefixes and all of the /24
components of them, probably due to a lack of consistent current
information about recommended routing configuration.
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9. Conclusions and Recommendations
In 1992, when CIDR was first developed, there were serious problems
facing the continued growth of the Internet. Growth in routing state
complexity, and the rapid increase in consumption of address space
made it appear that one or both problems would preclude continued
growth of the Internet within a few short years.
Deployment of CIDR, in combination with BGP4's support for carrying
classless prefix routes, alleviated the short-term crisis. It was
only through a concerted effort by both the equipment manufacturers
and the provider community that this was achieved. The threat (and,
perhaps in some cases, actual implementation of) charging networks
for advertising prefixes may have offered an additional incentive to
share the address space, and hence the associated costs of
advertising routes to service providers.
The IPv4 routing system architecture carries topology information
based on aggregate address advertisements and a collection of more-
specific advertisements that are associated with traffic engineering,
multi-homing and local configuration. As of March, 2005, the base
aggregate address load in the routing system has some 75,000 entries.
Approximately 85,000 additional entries are more specific entries of
this base "root" collection. There is reason to believe that many of
these additional entries are exist to solve problems of regional or
even local scope and should not need to be globally propagated.
An obvious question to ask is whether CIDR can continue to be a
viable approach to keeping global routing state growth and address
space depletion at sustainable rates. Recent measurements indicate
that exponential growth has resumed but further analysis suggests
that this trend can be mitigated by a more active effort to educate
service providers on efficient aggregation strategies and proper
equipment configuration. Looking farther forward, there is a clear
need for better multi-homing technology that does not require global
routing state for each site and for methods of performing traffic
load balancing that do not require adding even more state. Without
such developments and in the absence of major architectural change,
aggregation is the only tool available for making routing scale in
the global Internet.
10. Security Considerations
The introduction of routing protocols which support classless
prefixes and a move to a forwarding model that mandates that more-
specific (longest-match) routes be preferred when they overlap with
routes to less-specific prefixes introduces at least two security
concerns:
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Traffic can be hijacked by advertising a prefix for a given
destination that is more specific than the aggregate that is
normally advertised for that destination. For example, assume a
popular end system with address 192.168.17.100 that is connected
to a service provider that advertises 192.168.16.0/20. A
malicious network operator interested in intercepting traffic for
this site might advertise, or at least attempt to advertise,
192.168.17.0/24 into the global routing system. Because this
prefix is more-specific than the "normal" prefix, traffic will be
diverted away from the legitimate end system and to the network
owned by the malicious operator. Prior to the advent of CIDR, it
was possible to induce traffic from some parts of the network to
follow a false advertisement that exactly matched a particular
network number; CIDR makes this problem somewhat worse, since
longest-match routing generally causes all traffic to prefer more-
specific routes over less-specific routes. The remedy for the
CIDR-based attack, though, is the same as for a pre-CIDR-based
attack: establishment of trust relationships between providers,
coupled with and strong route policy filters at provider borders.
Unfortunately, the implementation of such filters is difficult in
the highly de-centralized Internet. As a workaround, many
providers do implement generic filters that set upper bounds,
derived from RIR guidelines for the sizes of blocks that they
allocate, on the lengths of prefixes that are accepted from other
providers. It is worth noting that "spammers" have been observed
using this sort of attack to temporarily hijack address space in
order to hide the origin of the traffic ("spam" email messages)
that they generate.
Denial-of-service attacks can be launched against many parts of
the Internet infrastructure by advertising a large number of
routes into the system. Such an attack is intended to cause
router failures by overflowing routing and forwarding tables. A
good example of a non-malicious incident which caused this sort of
failure was the infamous "AS 7007" event [7007] where a router
mis-configuration by an operator caused a huge number of invalid
routes to be propagated through the global routing system. Again,
this sort of attack is not really new with CIDR; using legacy
class A/B/C routes, it was possible to advertise a maximum of
16843008 unique network numbers into the global routing system, a
number which is sufficient to cause problems for even the most
modern routing equipment made in 2005. What is different is that
the moderate complexity of correctly configuring routers in the
presence of CIDR does tend to make accidental "attacks" of this
sort more likely. Measures to prevent this sort of attack are
much the same as those described above for the hijacking, with the
addition that best common practice is to also configure a
reasonable maximum number of prefixes that a border router will
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accept from its neighbors.
Note that this is not intended to be an exhaustive analysis of the
sorts of attacks that CIDR makes easier; a more comprehensive
analysis of security vulnerabilities in the global routing system
is beyond the scope of this document.
11. Acknowledgments
The authors wish to express appreciation to the other original
authors of RFC1519 (Kannan Varadhan, Jessica Yu), to the ROAD group
with whom many of the ideas behind CIDR were inspired and developed,
and to the early reviewers of this re-spun version of the document
(Barry Greene, Geoff Huston, Danny McPherson, Dave Meyer, Eliot Lear,
Bill Norton, Ted Seely, Philip Smith, Pekka Savola) whose comments,
corrections, and suggestions were invaluable.
12. References
12.1 Normative References
[RFC791] Postel, J., "Internet Protocol", RFC 791, September 1981.
12.2 Informative References
[RFC1338] Fuller, V., Li, T., Varadhan, K., and J. Yu,
"Supernetting: an Address Assignment and Aggregation
Strategy", RFC 1338, June 1992.
[RFC1519] Fuller, V., Li, T., Varadhan, K., and J. Yu, "Classless
Inter-Domain Routing: an Address Assignment and
Aggregation Strategy", RFC 1519, September 1993.
[IANA] "Internet Assigned Numbers Authority",
<http://www.iana.org>.
[RFC3221] Huston, G., "Commentary on Inter-Domain Routing in the
Internet", RFC 3221, December 2001.
[NRO] "Number Resource Organization", <http://www.nro.net>.
[RFC1380] Gross, P. and P. Almquist, "IESG Deliberations on Routing
and Addressing", RFC 1380, November 1992.
[LWRD] "The Long and Winding Road",
<http://rms46.vlsm.org/1/42.html>.
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[RFC2178] Moy, J., "The OSPF Specification Version 2", RFC 2178,
July 1997.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC2453] Malkin, G., "RIP Version 2", RFC 2453, November 1998.
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[RFC1058] Hedrick, C., "Routing Information Protocol", RFC 1058,
June 1988.
[RFC904] Mills, D., "Exterior Gateway Protocol formal
specification", RFC 904, April 1984.
[RFC3021] Retana, A., White, R., Fuller, V., and D. McPherson,
"Using 31-Bit Prefixes on IPv4 Point-to-Point Links",
RFC 3021, December 2000.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RIPE] "RIPE Network Coordination Centre", <http://www.ripe.net>.
[RFC1518] Rekhter, Y. and T. Li, "An Architecture for IP Address
Allocation with CIDR", RFC 1518, September 1993.
[RFC2317] Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
ADDR.ARPA delegation", RFC 2317, March 1998.
[CRPT] "The CIDR Report", <http://www.cidr-report.org/>.
[CBGP] "Graph: Active BGP Table Entries, 1988 to Present",
<http://bgp.potaroo.net/as4637/>.
[CPOL] "CIDR Police - Please Pull Over and Show Us Your BGP",
<http://www.nanog.org/mtg-0302/cidr.html>.
[7007] "NANOG mailing list discussion of the "AS 7007" incident",
<http://www.merit.edu/mail.archives/nanog/1997-04/
msg00340.html>.
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Authors' Addresses
Vince Fuller
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: vaf@cisco.com
Tony Li
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: tli@cisco.com
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Fuller & Li Expires December 12, 2005 [Page 28]
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