draft-ietf-pier-renum-ovrvw-01.txt   rfc2071.txt 
PIER Working Group P. Ferguson Network Working Group P. Ferguson
Internet Draft cisco Systems, Inc. Request for Comments: 2071 cisco Systems, Inc.
August 1996 H. Berkowitz Category: Informational H. Berkowitz
Expires in six months PSC International PSC International
January 1997
Network Renumbering Overview: Network Renumbering Overview:
Why would I want it and what is it anyway? Why would I want it and what is it anyway?
draft-ietf-pier-renum-ovrvw-01.txt
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Abstract Abstract
The PIER [Procedures for Internet/Enterprise Renumbering] working The PIER [Procedures for Internet/Enterprise Renumbering] working
group is compiling a series of documents to assist and instruct group is compiling a series of documents to assist and instruct
organizations in their efforts to renumber. However, it is becoming organizations in their efforts to renumber. However, it is becoming
apparent that, with the increasing number of new Internet Service apparent that, with the increasing number of new Internet Service
Providers (ISP's) and organizations getting connected to the Providers (ISP's) and organizations getting connected to the Internet
Internet for the first time, the concept of network renumbering for the first time, the concept of network renumbering needs to be
needs to be further defined. This document attempts to clearly further defined. This document attempts to clearly define the
define the concept of network renumbering and discuss some of the concept of network renumbering and discuss some of the more pertinent
more pertinent reasons why an organization would have a need to do reasons why an organization would have a need to do so.
so.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Network Renumbering Defined. . . . . . . . . . . . . . . . . 3 3. Network Renumbering Defined. . . . . . . . . . . . . . . . . 3
4. Reasons for Renumbering. . . . . . . . . . . . . . . . . . . 3 4. Reasons for Renumbering. . . . . . . . . . . . . . . . . . . 3
5. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . 12
7. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . 12 7. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 13 9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction 1. Introduction
The popularity of connecting to the global Internet over the course The popularity of connecting to the global Internet over the course
of the past several years has spawned new problems; what most people of the past several years has spawned new problems; what most people
casually refer to as ``growing pains'' can be attributed to more casually refer to as "growing pains" can be attributed to more basic
basic problems in understanding the requirements for Internet problems in understanding the requirements for Internet connectivity.
connectivity. However, the reasons why organizations may need to However, the reasons why organizations may need to renumber their
renumber their networks can greatly vary. We'll discuss these issues networks can greatly vary. We'll discuss these issues in some amount
in some amount of detail below. It is not within the intended scope of detail below. It is not within the intended scope of this
of this document to discuss renumbering methodologies, techniques, or document to discuss renumbering methodologies, techniques, or tools.
tools.
2. Background 2. Background
The ability for any network or interconnected devices, such as The ability for any network or interconnected devices, such as
desktop PCs or workstations, to obtain connectivity to any potential desktop PCs or workstations, to obtain connectivity to any potential
destination in the global Internet is reliant upon the possession of destination in the global Internet is reliant upon the possession of
unique IP host addresses [1]. A duplicate host address that is unique IP host addresses [1]. A duplicate host address that is being
being used elsewhere in the Internet could best be described as used elsewhere in the Internet could best be described as
problematic, since the presence of duplicate addresses would cause problematic, since the presence of duplicate addresses would cause
one of the destinations to be unreachable from some origins in the one of the destinations to be unreachable from some origins in the
Internet. It should be noted, however, that globally unique IP Internet. It should be noted, however, that globally unique IP
addresses are not always necessary, and is dependent on the addresses are not always necessary, and is dependent on the
connectivity requirements [2]. connectivity requirements [2].
However, the recent popularity in obtaining Internet connectivity However, the recent popularity in obtaining Internet connectivity has
has made these types of connectivity dependencies unpredictable, made these types of connectivity dependencies unpredictable, and
and conventional wisdom in the Internet community dictates that conventional wisdom in the Internet community dictates that the
the various address allocation registries, such as the interNIC, various address allocation registries, such as the InterNIC, as well
as well as the ISP's, become more prudent in their address as the ISP's, become more prudent in their address allocation
allocation strategies. In that vein, the interNIC has defined strategies. In that vein, the InterNIC has defined address
address allocation policies [3] wherein the majority of address allocation policies [3] wherein the majority of address allocations
allocations for end-user networks are accommodated by their for end-user networks are accommodated by their upstream ISP, except
upstream ISP, except in cases where dual- or multihoming and in cases where dual- or multihoming and very large blocks of
very large blocks of addresses are required. With this allocation addresses are required. With this allocation policy becoming
policy becoming standard current practice, it presents unique standard current practice, it presents unique problems regarding the
problems regarding the portability of addresses from one provider portability of addresses from one provider to another.
to another.
As a practical matter, end users cannot assume they ``own'' address As a practical matter, end users cannot assume they "own" address
allocations, if their intention is to be to have full connectivity allocations, if their intention is to be to have full connectivity to
to the global Internet. Rather, end users will ``borrow'' part of the the global Internet. Rather, end users will "borrow" part of the
address space of an upstream provider's allocation. The larger address space of an upstream provider's allocation. The larger
provider block from which their space is suballocated will have been provider block from which their space is suballocated will have been
assigned in a manner consistent with global Internet routing. assigned in a manner consistent with global Internet routing.
Not having ``permanent'' addresses does not mean users will not have Not having "permanent" addresses does not mean users will not have
unique identifiers. Such identifiers are typically Domain Name unique identifiers. Such identifiers are typically Domain Name System
System (DNS) [4] names for endpoints such as servers and (DNS) [4] names for endpoints such as servers and workstations.
workstations. Mechanisms such as the Dynamic Host Configuration Mechanisms such as the Dynamic Host Configuration Protocol (DHCP) [5]
Protocol (DHCP) [5] can help automate the assignment and maintenance can help automate the assignment and maintenance of host names, as
of host names, as well as the 'borrowed' addresses required for well as the 'borrowed' addresses required for routing-level
routing-level connectivity. connectivity.
The PIER Working Group is developing procedures and guidelines for The PIER Working Group is developing procedures and guidelines for
detailed renumbering of specific technologies, such as routers [6]. detailed renumbering of specific technologies, such as routers [6].
PIER WG documents are intended to suggest methods both for making PIER WG documents are intended to suggest methods both for making
existing networks prepared for convenient renumbering, as well as existing networks prepared for convenient renumbering, as well as for
for operational transition to new addressing schemes. operational transition to new addressing schemes.
Also, in many instances, organizations who have never connected to Also, in many instances, organizations who have never connected to
the Internet, yet have been using arbitrary blocks of addresses since the Internet, yet have been using arbitrary blocks of addresses since
their construction, have different and unique challenges. their construction, have different and unique challenges.
3. Network Renumbering Defined 3. Network Renumbering Defined
In the simplest of definitions, the exercise of renumbering a In the simplest of definitions, the exercise of renumbering a network
network consists of changing the IP host addresses, and perhaps consists of changing the IP host addresses, and perhaps the network
the network mask, of each device within the network that has an mask, of each device within the network that has an address
address associated with it. This activity may or may not consist associated with it. This activity may or may not consist of all
of all networks within a particular domain, such as FOO.EDU, or networks within a particular domain, such as FOO.EDU, or networks
networks which comprise an entire autonomous system. which comprise an entire autonomous system.
Devices which may need to be renumbered, for example, are networked Devices which may need to be renumbered, for example, are networked
PC's, workstations, printers, file servers, terminal servers, and PC's, workstations, printers, file servers, terminal servers, and
routers. While this is not an all-inclusive list, the PIER working routers. Renumbering a network may involve changing host parameters
group is making efforts to compile documentation to identify these and configuration files which contain IP addresses, such as
devices in a more detailed fashion. configuration files which contain addresses of DNS and other servers,
addresses contained in SNMP [7] management stations, and addresses
configured in access control lists. While this is not an all-
inclusive list, the PIER working group is making efforts to compile
documentation to identify these devices in a more detailed fashion.
Network renumbering need not be sudden activity, either; in most Network renumbering need not be sudden activity, either; in most
instances, an organization's upstream service provider(s) will instances, an organization's upstream service provider(s) will allow
allow a grace period where both the ``old'' addresses and the ``new'' a grace period where both the "old" addresses and the "new" addresses
addresses may be used in parallel. may be used in parallel.
4. Reasons for Renumbering 4. Reasons for Renumbering
The following sections discuss particular reasons which may The following sections discuss particular reasons which may
precipitate network renumbering, and are not presented in any precipitate network renumbering, and are not presented in any
particular order of precedence. They are grouped into reasons that particular order of precedence. They are grouped into reasons that
primarily reflect decisions made in the past, operational primarily reflect decisions made in the past, operational
requirements of the present, or plans for the future. requirements of the present, or plans for the future.
Some of these requirements reflect evolution in the organization's Some of these requirements reflect evolution in the organization's
mission, such as a need to communicate with business partners, or mission, such as a need to communicate with business partners, or to
to work efficiently in a global Internet. Other requirements work efficiently in a global Internet. Other requirements reflect
reflect changes in network technologies. changes in network technologies.
4.1 Past 4.1 Past
Many organizations implemented IP-based networks not for Many organizations implemented IP-based networks not for connectivity
connectivity to the Internet, but simply to make use of effective to the Internet, but simply to make use of effective data
data communications mechanisms. These organizations subsequently communications mechanisms. These organizations subsequently found
found valid reasons to connect to other organizations or the valid reasons to connect to other organizations or the Internet in
Internet in general, but found the address structures they chose general, but found the address structures they chose incompatible
incompatible with overall Internet practice. with overall Internet practice.
Other organizations connected early to the Internet, but did so at Other organizations connected early to the Internet, but did so at a
a time when address space was not scarce. Yet other organizations time when address space was not scarce. Yet other organizations
still have no requirement to connect to the Internet, but have still have no requirement to connect to the Internet, but have legacy
legacy addressing structures that do not scale to adequate size. addressing structures that do not scale to adequate size.
4.1.1 Initial addressing using non-unique addresses 4.1.1 Initial addressing using non-unique addresses
As recently as two years ago, many organizations had no intention As recently as two years ago, many organizations had no intention of
of connecting to the Internet, and constructed their corporate or connecting to the Internet, and constructed their corporate or
organizational network(s) using unregistered, non-unique network organizational network(s) using unregistered, non-unique network
addresses. Obviously, as most problems evolve, these same addresses. Obviously, as most problems evolve, these same
organizations determined that Internet connectivity had become organizations determined that Internet connectivity had become a
a valuable asset, and subsequently discovered that they could no valuable asset, and subsequently discovered that they could no longer
longer use the same unregistered, non-unique network addresses use the same unregistered, non-unique network addresses that were
that were previously deployed throughout their organization. previously deployed throughout their organization. Thus, the labor
Thus, the labor of renumbering to valid network addresses is of renumbering to valid network addresses is now upon them, as they
now upon them, as they move to connect to the global Internet. move to connect to the global Internet.
While obtaining valid, unique addresses are certainly required While obtaining valid, unique addresses is certainly required to
to obtain full Internet connectivity in most circumstances, the obtain full Internet connectivity in most circumstances, the number
number of unique addresses required can be significantly reduced of unique addresses required can be significantly reduced by the
by the implementation of Network Address Translation (NAT) devices implementation of Network Address Translation (NAT) devices [8] and
[7] and the use of private address space, as specified in [8]. the use of private address space, as specified in [9]. NAT reduces
NAT reduces not only the number of required unique addresses, but not only the number of required unique addresses, but also localizes
also localizes the changes required by renumbering. the changes required by renumbering.
It should also be noted that NAT technology may not always be It should also be noted that NAT technology may not always be a
a viable option, depending upon scale of addressing, performance viable option, depending upon scale of addressing, performance or
or topological constraints. topological constraints.
4.1.2 Legacy address allocation 4.1.2 Legacy address allocation
There are also several instances where organizations were originally There are also several instances where organizations were originally
allocated very large amounts of address space, such as traditional allocated very large amounts of address space, such as traditional
``Class A'' or ``Class B'' allocations, while the actual address "Class A" or "Class B" allocations, while the actual address
requirements are much less than the total amount of address space requirements are much less than the total amount of address space
originally allocated. In many cases, these organizations could originally allocated. In many cases, these organizations could
suffice with a smaller CIDR allocation, and utilize the allocated suffice with a smaller CIDR allocation, and utilize the allocated
address space in a more efficient manner. As allocation requirements address space in a more efficient manner. As allocation requirements
become more stringent, mechanisms to review how these organizations become more stringent, mechanisms to review how these organizations
are utilizing their address space could, quite possibly, result in are utilizing their address space could, quite possibly, result in a
a request to return the original allocation to a particular registry request to return the original allocation to a particular registry
and renumber with a more appropriately sized address block. and renumber with a more appropriately sized address block.
4.1.3 Limitations of Bridged Internetworks 4.1.3 Limitations of Bridged Internetworks
Bridging has a long and distinguished history in legacy networks. Bridging has a long and distinguished history in legacy networks. As
As networks grow, however, traditional bridged networks reach networks grow, however, traditional bridged networks reach
performance- and stability-related limits, including (but not performance- and stability-related limits, including (but not limited
limited to) broadcast storms. to) broadcast storms.
Early routers did not have the speed to handle the needs of some Early routers did not have the speed to handle the needs of some
large networks. Some organizations were literally not able to move large networks. Some organizations were literally not able to move
to routers until router forwarding performance improved to be to routers until router forwarding performance improved to be
comparable to bridges. Now that routers are of comparable or comparable to bridges. Now that routers are of comparable or
superior speed, and offer more robust features, replacing bridged superior speed, and offer more robust features, replacing bridged
networks becomes reasonable. networks becomes reasonable.
IP addresses assigned to pure bridged networks tend not to be IP addresses assigned to pure bridged networks tend not to be
subnetted, yet subnetting is a basic approach for router networks. subnetted, yet subnetting is a basic approach for router networks.
Introducing subnetting is a practical necessity in moving from Introducing subnetting is a practical necessity in moving from
bridging to routing. bridging to routing.
Special cases of bridging are realized in workgroup switching Special cases of bridging are realized in workgroup switching
systems, discussed below. systems, discussed below.
4.1.4 Limitations of Legacy Routing Systems 4.1.4 Limitations of Legacy Routing Systems
Other performance problems might come from routing mechanisms that Other performance problems might come from routing mechanisms that
advertise excessive numbers of routing updates (e.g., RIP, IGRP). advertise excessive numbers of routing updates (e.g., RIP, IGRP).
Appropriate replacement protocols (e.g., OSPF, EIGRP, IS-IS) will Likewise, appropriate replacement protocols (e.g., OSPF, EIGRP, S-IS)
work best with a structured addressing system that encourages will work best with a structured addressing system that encourages
aggregation. aggregation.
4.1.5 Limitations of System Administration Methodologies 4.1.5 Limitations of System Administration Methodologies
There can be operational limits to growth based on the difficulty There can be operational limits to growth based on the difficulty of
of adds, moves and changes. As enterprise networks grow, it may adds, moves and changes. As enterprise networks grow, it may be
be necessary to delegate portions of address assignment and necessary to delegate portions of address assignment and maintenance.
maintenance. If address space has been assigned randomly or If address space has been assigned randomly or inefficiently, it may
inefficiently, it may be difficult to delegate portions of the be difficult to delegate portions of the address space.
address space.
It is not unusual for organizational networks to grow sporadically, It is not unusual for organizational networks to grow sporadically,
obtaining an address prefix here and there, in a non-contiguous obtaining an address prefix here and there, in a non-contiguous
fashion. Depending on the number of prefixes that an organization fashion. Depending on the number of prefixes that an organization
acquires over time, it may become increasingly unmanageable or demand acquires over time, it may become increasingly unmanageable or demand
higher levels of maintenance and administration when individual higher levels of maintenance and administration when individual
prefixes are acquired in this way. prefixes are acquired in this way.
Reasonable IP address management may in general simplify continuing Reasonable IP address management may in general simplify continuing
system administration; a good numbering plan is also a good system administration; a good numbering plan is also a good
renumbering plan. Renumbering may force a discipline into system renumbering plan. Renumbering may force a discipline into system
administration that will reduce long-term support costs. administration that will reduce long-term support costs.
It has been observed ``...there is no way to renumber a network It has been observed "...there is no way to renumber a network
without an inventory of the hosts (absent DHCP). On a large without an inventory of the hosts (absent DHCP). On a large network
network that needs a database, plus tools and staff to that needs a database, plus tools and staff to maintain the
maintain the database.''[9] It can be argued that a detailed database."[10] It can be argued that a detailed inventory of router
inventory of router configurations is even more essential. configurations is even more essential.
4.2 Present 4.2 Present
Organizations now face needs to connect to the global Internet, or Organizations now face needs to connect to the global Internet, or at
at a minimum to other organizations through bilateral private links. a minimum to other organizations through bilateral private links.
Certain new transmission technologies have tended to redefine the Certain new transmission technologies have tended to redefine the
basic notion of an IP subnet. An IP numbering plan needs to work basic notion of an IP subnet. An IP numbering plan needs to work
with these new ideas. Legacy bridged networks and leading-edge with these new ideas. Legacy bridged networks and leading-edge
workgroup switched networks may very well need changes in the workgroup switched networks may very well need changes in the
subnetting structure. Renumbering needs may also develop due to subnetting structure. Renumbering needs may also develop due to the
the characteristics of new WAN technologies, especially nonbroadcast characteristics of new WAN technologies, especially nonbroadcast
multiaccess (NBMA) services such as Frame-Relay and Asynchronous multi-access (NBMA) services such as Frame-Relay and Asynchronous
Transfer Mode (ATM). Transfer Mode (ATM).
Increased use of telecommuting by mobile workers, and in small and Increased use of telecommuting by mobile workers, and in small and
home offices, need on-demand WAN connectivity, using modems or ISDN. home offices, need on-demand WAN connectivity, using modems or ISDN.
Effective use of demand media often requires changes in numbering Effective use of demand media often requires changes in numbering and
and routing. routing.
4.2.1 Change in organizational structure or network topology 4.2.1 Change in organizational structure or network topology
As companies grow, through mergers, acquisitions and reorganizations, As companies grow, through mergers, acquisitions and reorganizations,
the need may arise for realignment and modification of the various the need may arise for realignment and modification of the various
organizational network architectures. The connectivity of disparate organizational network architectures. The connectivity of disparate
corporate networks present unique challenges in the realm of corporate networks present unique challenges in the realm of
renumbering, since one or more individual networks may have to be renumbering, since one or more individual networks may have to be
blended into a much larger architecture consisting a different IP blended into a much larger architecture consisting a different IP
address prefix altogether. address prefix altogether.
skipping to change at page 7, line 14 skipping to change at page 7, line 32
In such cases, one or both organizations may need to renumber into In such cases, one or both organizations may need to renumber into
different parts of the private address space, or obtain unique different parts of the private address space, or obtain unique
registered addresses. registered addresses.
4.2.3 Change of Internet Service Provider 4.2.3 Change of Internet Service Provider
As mentioned previously in Section 2, it is increasingly becoming As mentioned previously in Section 2, it is increasingly becoming
current practice for organizations to have their IP addresses current practice for organizations to have their IP addresses
allocated by their upstream ISP. Also, with the advent of Classless allocated by their upstream ISP. Also, with the advent of Classless
Inter Domain Routing (CIDR) [10], and the considerable growth in the Inter Domain Routing (CIDR) [11], and the considerable growth in the
size of the global Internet table, Internet Service Providers size of the global Internet table, Internet Service Providers are
are becoming more and more reluctant to allow customers to continue becoming more and more reluctant to allow customers to continue using
using addresses which were allocated by the ISP, when the customer addresses which were allocated by the ISP, when the customer
terminates service and moves to another ISP. The prevailing terminates service and moves to another ISP. The prevailing reason
reason is that the ISP was previously issued a CIDR block of is that the ISP was previously issued a CIDR block of contiguous
contiguous address space, which can be announced to the remainder of address space, which can be announced to the remainder of the
the Internet community as a single prefix. (A prefix is what is Internet community as a single prefix. (A prefix is what is referred
referred to in classless terms as a contiguous block of IP to in classless terms as a contiguous block of IP addresses.) If a
addresses.) If a non-customer advertises a specific component non-customer advertises a specific component of the CIDR block, then
of the CIDR block, then this adds an additional routing entry to this adds an additional routing entry to the global Internet routing
the global Internet routing table. This is what is commonly table. This is what is commonly referred to as "punching holes" in a
referred to as ``punching holes'' in a CIDR block. Consequently, CIDR block. Consequently, there are usually no routing anomalies in
there are usually no routing anomalies in doing this since a specific doing this since a specific prefix is always preferred over an
prefix is always preferred over an aggregate route. However, if aggregate route. However, if this practice were to happen on a large
this practice were to happen on a large scale, the growth of the scale, the growth of the global routing table would become much
global routing table would become much larger, and perhaps too larger, and perhaps too large for current backbone routers to
large for current backbone routers to accommodate in an acceptable accommodate in an acceptable fashion with regards to performance of
fashion with regards to performance of recalculating routing recalculating routing information and sheer size of the routing table
information and sheer size of the routing table itself. For obvious itself. For obvious reasons, this practice is highly discouraged by
reasons, this practice is highly discouraged by ISP's with CIDR ISP's with CIDR blocks, and some ISP's are making this a contractual
blocks, and some ISP's are making this a contractual issue, so that issue, so that customers understand that addresses allocated by the
customers understand that addresses allocated by the ISP are non- ISP are non-portable.
portable.
It is noteworthy to mention that the likelihood of being forced to It is noteworthy to mention that the likelihood of being forced to
renumber in this situation is inversely proportional to the size of renumber in this situation is inversely proportional to the size of
the customer's address space. For example, an organization with a the customer's address space. For example, an organization with a
/16 allocation may be allowed to consider the address space /16 allocation may be allowed to consider the address space
``portable'', while an organization with multiple non-contiguous "portable", while an organization with multiple non-contiguous /24
/24 allocations may not. While the scenarios may be vastly different allocations may not. While the scenarios may be vastly different in
in scope, it becomes an issue to be decided at the discretion of the scope, it becomes an issue to be decided at the discretion of the
initial allocating entity, and the ISP's involved; the major deciding initial allocating entity, and the ISP's involved; the major deciding
factor being whether or not the change will fragment an existing CIDR factor being whether or not the change will fragment an existing CIDR
block and whether it will significantly contribute to the overall block and whether it will significantly contribute to the overall
growth of the global Internet routing tables. growth of the global Internet routing tables.
It should also be noted that (contrary to opinions sometimes voiced) It should also be noted that (contrary to opinions sometimes voiced)
this form of renumbering is a technically necessary consequence of this form of renumbering is a technically necessary consequence of
changing ISP's, rather than a commercial or political mandate. changing ISP's, rather than a commercial or political mandate.
4.2.3 Internet Global Routing 4.2.3 Internet Global Routing
Even large organizations, now connected to the Internet with Even large organizations, now connected to the Internet with
``portable'' address space, may find their address allocation too "portable" address space, may find their address allocation too
small. Current registry guidelines require that address space usage small. Current registry guidelines require that address space usage
be justified by an engineering plan. Older networks may not have be justified by an engineering plan. Older networks may not have
efficiently utilized existing address space, and may need to make efficiently utilized existing address space, and may need to make
their existing structures more efficient before new address their existing structures more efficient before new address
allocations can be made. allocations can be made.
4.2.4 Internal Use of LAN Switching 4.2.4 Internal Use of LAN Switching
Introducing workgroup switches may introduce subtle renumbering Introducing workgroup switches may introduce subtle renumbering
needs. Fundamentally, workgroup switches are specialized, high- needs. Fundamentally, workgroup switches are specialized, high-
performance bridges, which make their main forwarding decisions performance bridges, which make their main forwarding decisions based
based on Layer 2 (MAC) address information. Even so, they rarely on Layer 2 (MAC) address information. Even so, they rarely are
are independent of Layer 3 (IP) address structure. Pure Layer 2 independent of Layer 3 (IP) address structure. Pure Layer 2
switching has a ``flat'' address space that will need to be switching has a "flat" address space that will need to be renumbered
renumbered into a hierarchical, subnetted space consistent with into a hierarchical, subnetted space consistent with routing.
routing.
Introducing single switches or stacks of switches may not have Introducing single switches or stacks of switches may not have
significant impact on addressing, as long as it is understood significant impact on addressing, as long as it is understood that
that each system of switches is a single broadcast domain. Each each system of switches is a single broadcast domain. Each broadcast
broadcast domain should map to a single IP subnetwork. domain should map to a single IP subnetwork.
Virtual LANs (VLANs) further extend the complexity of the role of Virtual LANs (VLANs) further extend the complexity of the role of
workgroup switches. It is generally true that moving an end workgroup switches. It is generally true that moving an end station
station from one switch port to another within the same ``color'' from one switch port to another within the same VLAN will not cause
VLAN will not cause major changes in addressing. Many overview major changes in addressing. Many overview presentations of this
presentations of this technology do not make it clear that moving technology do not make it clear that moving the same end station
the same end station between different colors will move the between different VLANs will move the end station into another IP
end station into another IP subnet, requiring a significant subnet, requiring a significant address change.
address change.
Switches are commonly managed by SNMP applications. These Switches are commonly managed by SNMP applications. These network
network management applications communicate with managed devices management applications communicate with managed devices using IP.
using IP. Even if the switch does not do IP forwarding, it will Even if the switch does not do IP forwarding, it will itself need IP
itself need IP addresses if it is to be managed. Also, if the addresses if it is to be managed. Also, if the clients and servers in
clients and servers in the workgroup are managed by SNMP, they the workgroup are managed by SNMP, they will also require IP
will also require IP addresses. The workgroup, therefore, will addresses. The workgroup, therefore, will need to appear as one or
need to appear as one or more IP subnetworks. more IP subnetworks.
Increasingly, internetworking products are not purely Layer 2 or Increasingly, internetworking products are not purely Layer 2 or
Layer 3 devices. A workgroup switch product often includes a routing Layer 3 devices. A workgroup switch product often includes a routing
function, so the numbering plan must support both flat Layer 2 and function, so the numbering plan must support both flat Layer 2 and
hierarchical Layer 3 addressing. hierarchical Layer 3 addressing.
4.2.4 Internal Use of NBMA Cloud Services 4.2.4 Internal Use of NBMA Cloud Services
"Cloud" services such as frame relay often are more economical than "Cloud" services such as frame relay often are more economical than
traditional services. At first glance, when converting existing traditional services. At first glance, when converting existing
enterprise networks to NBMA, it might appear that the existing subnet enterprise networks to NBMA, it might appear that the existing subnet
structure should be preserved, but this is often not the case. structure should be preserved, but this is often not the case.
Many organizations often began by treating the "cloud" as a single Many organizations often began by treating the "cloud" as a single
subnet, but experience has shown it is often better to treat the subnet, but experience has shown it is often better to treat the
individual virtual circuits as separate subnets, which appear as individual virtual circuits as separate subnets, which appear as
traditional point-to-point circuits. When the individual traditional point-to-point circuits. When the individual point-to-
point-to-point VCs become separate subnets, efficient address point VCs become separate subnets, efficient address utilization
utilization requires the use of long prefixes (i.e., 30 bit) for requires the use of long prefixes (i.e., 30 bit) for these subnets.
these subnets. In practice, obtaining 30 bit prefixes means the In practice, obtaining 30 bit prefixes means the logical network
logical network should support variable length subnet masks (VLSM). should support variable length subnet masks (VLSM). VLSMs are the
VLSMs are the primary method in which an assigned prefix can be primary method in which an assigned prefix can be subnetted
subnetted efficiently for different media types. This is efficiently for different media types. This is accomplished by
accomplished by establishing one or more prefix lengths for LAN establishing one or more prefix lengths for LAN media with more than
media with more than two hosts, and subdividing one or more of these two hosts, and subdividing one or more of these shorter prefixes into
shorter prefixes into longer /30 prefixes that minimize address loss. longer /30 prefixes that minimize address loss.
There are alternative ways to configure routing over NBMA, using There are alternative ways to configure routing over NBMA, using
special mechanisms to exploit or simulate point-to-multipoint VCs. special mechanisms to exploit or simulate point-to-multipoint VCs.
These often have a significant performance impact, and may be less These often have a significant performance impact, and may be less
reliable because a single routing point of failure is created. reliable because a single routing point of failure is created.
Motivations for such alternatives tend to include: Motivations for such alternatives tend to include:
1. A desire not to use VLSM. This is often founded in fear 1. A desire not to use VLSM. This is often founded in fear
rather than technology. rather than technology.
2. Router implementation issues that limit the number of subnets 2. Router implementation issues that limit the number of subnets
or interfaces a given router can support. or interfaces a given router can support.
3. An inherently point-to-multipoint application (e.g., remote 3. An inherently point-to-multipoint application (e.g., remote
hosts to a data center). In such cases, some of the hosts to a data center). In such cases, some of the
limitations are due to the dynamic routing protocol in use. limitations are due to the dynamic routing protocol in use.
In such ``hub-and-spoke'' implementations, static routing can In such "hub-and-spoke" implementations, static routing can
be preferable from a performance and flexibility standpoint, be preferable from a performance and flexibility standpoint,
since it does not produce routing protocol chatter and is since it does not produce routing protocol chatter and is
unaffected by split horizon constraints. unaffected by split horizon constraints (namely, the inability
to build an adjacency with a peer within the same IP
subnetwork).
4.2.5 Expansion of Dialup Services 4.2.5 Expansion of Dialup Services
Dialup services, especially public Internet access providers, are Dialup services, especially public Internet access providers, are
experiencing explosive growth. This success represents a particular experiencing explosive growth. This success represents a particular
drain on the available address space, especially with a commonly drain on the available address space, especially with a commonly used
used practice of assigning unique addresses to each customer. practice of assigning unique addresses to each customer.
In this case, individual users announce their address to the In this case, individual users announce their address to the access
access server using PPP's IP control protocol (IPCP) [11]. The server using PPP's IP control protocol (IPCP) [12]. The server may
server may validate the proposed address against some type validate the proposed address against some type of user
of user identification, or simply make the address active in a identification, or simply make the address active in a subnet to
subnet to which the access server (or set of bridged access which the access server (or set of bridged access servers) belongs.
servers) belongs.
The preferred technique is to allocate dynamic addresses to the The preferred technique is to allocate dynamic addresses to the user
user from a pool of addresses available to the access server. from a pool of addresses available to the access server.
4.2.6 Returning segregate prefixes for an aggregate 4.2.6 Returning non-contiguous prefixes for an aggregate
In many instances, an organization can return their current, In many instances, an organization can return their current, non-
non-contiguous prefix allocations for a contiguous block of address contiguous prefix allocations for a contiguous block of address space
space of equal or greater size, which can be accommodated with CIDR. of equal or greater size, which can be accommodated with CIDR. Also,
Also, many organizations have begun to deploy classless interior many organizations have begun to deploy classless interior routing
routing protocols within their domains that make use of route protocols within their domains that make use of route summarization
summarization and other optimized routing features, effectively and other optimized routing features, effectively reducing the total
reducing the total number of routes being propagated within their number of routes being propagated within their internal network(s),
internal network(s), and making it much easier to administer and and making it much easier to administer and maintain.
maintain.
Hierarchical routing protocols such as OSPF scale best when the Hierarchical routing protocols such as OSPF scale best when the
address assignment of a given network reflects the topology, and the address assignment of a given network reflects the topology, and the
topology of the network can often be fluid. Given that the network is topology of the network can often be fluid. Given that the network is
fluid, even the best planned address assignment scheme, given time, fluid, even the best planned address assignment scheme, given time,
will diverge from the actual topology. While not required, some will diverge from the actual topology. While not required, some
organization may choose to gain the benefit of both technical and organization may choose to gain the benefit of both technical and
administrative scalability of their IGP by periodically renumbering administrative scalability of their IGP by periodically renumbering
to have address assignments reflect the network topology. Patrick to have address assignments reflect the network topology. Patrick
Henry once said ``the tree of liberty must from time to time be Henry once said "the tree of liberty must from time to time be
watered with the blood of patriots.'' In the Internet, routing watered with the blood of patriots." In the Internet, routing trees
trees of the best-planned networks need from time to time be of the best-planned networks need from time to time be watered with
watered with at least the sweat of network administrators. at least the sweat of network administrators. Improving aggregation
Improving aggregation is also highly encouraged to reduce the size is also highly encouraged to reduce the size of not only the global
of not only the global Internet routing table, but also the size Internet routing table, but also the size and scalability of interior
and scalability of interior routing within the enterprise. routing within the enterprise.
4.3 Future 4.3 Future
Emerging new protocols will most definitely affect addressing plans Emerging new protocols will most definitely affect addressing plans
and numbering schemes. and numbering schemes.
4.3.1 Internal Use of Switched Virtual Circuit Services 4.3.1 Internal Use of Switched Virtual Circuit Services
Services such as ATM virtual circuits, switched frame relay, etc., Services such as ATM virtual circuits, switched frame relay, etc.,
present challenges not considered in the original IP design. The present challenges not considered in the original IP design. The
basic IP decision in forwarding a packet is whether the destination basic IP decision in forwarding a packet is whether the destination
is local or remote, in relation to the source host's subnet. Address is local or remote, in relation to the source host's subnet. Address
resolution mechanisms are used to find the medium address of the resolution mechanisms are used to find the medium address of the
destination in the case of local destinations, or to find the medium destination in the case of local destinations, or to find the medium
address of the router in the case of remote routers. address of the router in the case of remote routers.
In these new services, there are cases where it is far more effective In these new services, there are cases where it is far more effective
to ``cut-through'' a new virtual circuit to the destination. If the to "cut-through" a new virtual circuit to the destination. If the
destination is on a different subnet than the source, the cut-through destination is on a different subnet than the source, the cut-through
typically is to the egress router that serves the destination subnet. typically is to the egress router that serves the destination subnet.
The advantage of cut-through in such a case is that it avoids the The advantage of cut-through in such a case is that it avoids the
latency of multiple router hops, and reduces load on ``backbone'' latency of multiple router hops, and reduces load on "backbone"
routers. The cut-through decision is usually made by an entry router routers. The cut-through decision is usually made by an entry router
that is aware of both the routed and switched environments. that is aware of both the routed and switched environments.
This entry router communicates with a address resolution server using This entry router communicates with a address resolution server using
the Next Hop Resolution Protocol (NHRP) [12]. This server maps the the Next Hop Resolution Protocol (NHRP) [13]. This server maps the
destination network address to either a next-hop router (where destination network address to either a next-hop router (where cut-
cut-through is not appropriate) or to an egress router reached over through is not appropriate) or to an egress router reached over the
the switched service. Obviously, the data base in such a server may switched service. Obviously, the data base in such a server may be
be affected by renumbering. Clients may have a hard-coded address affected by renumbering. Clients may have a hard-coded address of the
of the server, which again may need to change. server, which again may need to change. While the NHRP protocol
While the NHRP protocol specifications are still evolving at the specifications are still evolving at the time of this writing,
time of this writing, commercial implementations based on drafts commercial implementations based on drafts of the protocol standard
of the protocol standard are in use. are in use.
4.3.2 Transitioning to IP version 6 4.3.2 Transitioning to IP version 6
Of course, when IPv6 [13] deployment is set in motion, and as Of course, when IPv6 [14] deployment is set in motion, and as
methodologies are developed to transition to IPv6, renumbering will methodologies are developed to transition to IPv6, renumbering will
also be necessary, but perhaps not immediately mandatory. To aid also be necessary, but perhaps not immediately mandatory. To aid in
in the transition to IPv6, mechanisms to deploy dual- IPv4/IPv6 the transition to IPv6, mechanisms to deploy dual- IPv4/IPv6 stacks
stacks on network hosts should also become available. It is also on network hosts should also become available. It is also envisioned
envisioned that Network Address Translation (NAT) devices will be that Network Address Translation (NAT) devices will be developed to
developed to assist in the IPv4 to IPv6 transition, or perhaps assist in the IPv4 to IPv6 transition, or perhaps supplant the need
supplant the need to renumber the majority of interior networks to renumber the majority of interior networks altogether, but that is
altogether, but that is beyond the scope of this document. At the beyond the scope of this document. At the very least, DNS hosts will
very least, DNS hosts will need to be reconfigured to resolve new need to be reconfigured to resolve new host names and addresses, and
host names and addresses, and routers will need to be reconfigured routers will need to be reconfigured to advertise new prefixes.
to advertise new prefixes.
IPv6 address allocation will be managed by the Internet Assigned IPv6 address allocation will be managed by the Internet Assigned
Numbers Authority (IANA) as set forth in [14]. Numbers Authority (IANA) as set forth in [15].
5. Summary 5. Summary
As indicated by the Internet Architecture Board (IAB) in [15], As indicated by the Internet Architecture Board (IAB) in [16], the
the task of renumbering networks is becoming more widespread task of renumbering networks is becoming more widespread and
and commonplace. Although there are numerous reasons why an commonplace. Although there are numerous reasons why an organization
organization would desire, or be required to renumber, there are would desire, or be required to renumber, there are equally as many
equally as many reasons why address allocation should be done with reasons why address allocation should be done with great care and
great care and forethought at the onset, in order to minimize the forethought at the onset, in order to minimize the impact that
impact that renumbering would have on the organization. Even renumbering would have on the organization. Even with the most
with the most forethought and vision, however, an organization forethought and vision, however, an organization cannot foresee the
cannot foresee the possibility for renumbering. The best advice, possibility for renumbering. The best advice, in this case, is to be
in this case, is to be prepared, and get ready for renumbering. prepared, and get ready for renumbering.
6. Security Considerations 6. Security Considerations
Although no obvious security issues are discussed in this Although no obvious security issues are discussed in this document,
document, it stands to reason that renumbering certain devices it stands to reason that renumbering certain devices can defeat
can defeat security systems designed and based on static IP host security systems designed and based on static IP host addresses.
addresses. Care should be exercised by the renumbering entity Care should be exercised by the renumbering entity to ensure that all
to ensure that all security systems deployed with the network(s) security systems deployed with the network(s) which may need to be
which may need to be renumbered be given special consideration renumbered be given special consideration and significant forethought
and significant forethought to provide continued functionality to provide continued functionality and adequate security.
and adequate security.
7. Acknowledgments 7. Acknowledgments
Special acknowledgments to Yakov Rekhter [cisco Systems, Inc.], Special acknowledgments to Yakov Rekhter [cisco Systems, Inc.], Tony
Tony Bates [cisco Systems, Inc.] and Brian Carpenter [CERN] for Bates [cisco Systems, Inc.] and Brian Carpenter [CERN] for their
their contributions and editorial critique. contributions and editorial critique.
8. References 8. References
[1] RFC-1814, ``Unique Addresses are Good''; E. Gerich; IAB; July 1995 [1] Gerich, E., "Unique Addresses are Good", RFC 1814, IAB, July 1995.
[2] RFC-1775, ``To Be `On' the Internet''; D. Crocker, March 1995 [2] Crocker, D., "To Be `On' the Internet", RFC 1775, March 1995.
[3] Work in Progress; ``INTERNET REGISTRY IP ALLOCATION GUIDELINES''; [3] Hubbard, K., Kosters, M., Conrad, D., Karrenberg, D., and J.
K. Hubbard, J. Postel, M. Kosters, D. Conrad, D. Karrenberg; Postel, "INTERNET REGISTRY IP ALLOCATION GUIDELINES",
August 1996; draft-hubbard-registry-guidelines-05.txt BCP 12, RFC 2050, November 1996.
[4] RFC-1034, ``Domain Names - Concepts and Facilities''; [4] Mockapetris, P., "Domain Names - Concepts and Facilities",
P. Mockapetris, November 1987; and "Domain Names - Implementation and Specification",
RFC-1035, ``Domain Names - Implementation and Specification''; STD 13, RFCs 1034, 1035, November 1987.
P. Mockapetris, November 1987
[5] RFC-1541, ``Dynamic Host Configuration Protocol''; R. Droms, [5] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
October 1993 October 1993.
[6] Work in Progress, ``Router Renumbering Guide''; H. Berkowitz; [6] Berkowitz, H., "Router Renumbering Guide", RFC 2072,
June 1996; draft-ietf-pier-rr-01.txt January 1997.
[7] RFC-1631, ``The IP Network Address Translator (NAT)''; K. Egevang, [7] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "A Simple
P. Francis; May 1994 Network Management Protocol (SNMP)", STD 15, RFC 1157,
May 1990.
[8] RFC-1918, ``Address Allocation for Private Internets''; Y. Rekhter, [8] Egevang,, K., and P. Francis, "The IP Network Address Translator
R. Moskowitz, D. Karrenberg, G. de Groot, E. Lear; February 1996 (NAT)", RFC 1631, May 1994.
[9] Messages to PIER list on CERN renumbering; Brian Carpenter, CERN. [9] Rekhter, Y., Moskowitz, R., Karrenberg, D., de Groot, G-J., and E.
Available in PIER WG mailing list archives. Lear, "Address Allocation for Private Internets", RFC 1918,
February 1996.
[10] RFC-1519, ``Classless Inter-Domain Routing (CIDR): an Address [10] Messages to PIER list on CERN renumbering; Brian Carpenter, CERN.
Assignment and Aggregation Strategy''; V. Fuller, T. Li, J. Yu, Available in PIER WG mailing list archives.
K. Varadhan; October 1993
[11] RFC-1332, ``The PPP Internet Protocol Control Protocol (IPCP)''; [11] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
G. McGregor; May 1992 Inter-Domain Routing (CIDR): an Address Assignment and
Aggregation Strategy", RFC 1519, October 1993.
[12] Work in Progress; ``NBMA Next Hop Resolution Protocol (NHRP)''; [12] McGregor, G., "The PPP Internet Protocol Control Protocol
J. Luciani, D. Katz, D. Piscitello, B. Cole; July 1996; (IPCP)", RFC 1332, May 1992.
draft-ietf-rolc-nhrp-09.txt
[13] RFC-1883, ``Internet Protocol, Version 6 (IPv6) Specification''; [13] Luciani, J., Katz, D., Piscitello, D., and Cole, B., "NBMA Next
S. Deering, R. Hinden; December 1995 Hop Resolution Protocol (NHRP)", Work in Progress.
[14] RFC-1881, ``IPv6 Address Allocation Management''; IAB + IESG; [14] Deering, S., and R. Hinden, "Internet Protocol, Version 6 (IPv6)
December 1995 Specification", RFC 1883, December 1995.
[15] RFC-1900, ``Renumbering Needs Work''; B. Carpenter, Y. Rekhter; [15] IAB and IESG, "IPv6 Address Allocation Management", RFC 1881,
IAB; February 1996 December 1995.
9. Author's Address [16] Carpenter, B., and Y. Rekhter, "Renumbering Needs Work", RFC 1900,
February 1996.
9. Authors' Addresses
Paul Ferguson Paul Ferguson
cisco Systems, Inc. cisco Systems, Inc.
1875 Campus Commons Road 1875 Campus Commons Road
Suite 210 Suite 210
Reston, VA 22091 Reston, VA 22091
Phone: (703) 716-9538 Phone: (703) 716-9538
Fax: (703) 716-9599 Fax: (703) 716-9599
EMail: pferguso@cisco.com EMail: pferguso@cisco.com
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