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Network Working Group                                   Walter D. Lazear
INTERNET DRAFT                                         MITRE Corporation
                                                           February 1998
                                                     Expires August 1998

                      The Server Range Option for DHCP

Status of this memo

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1  Abstract

   This document describes a configuration option that may be used
   by hosts acting as IP forwarders.  The option contains information
   about the networks adjacent to the host and may be required by
   routing protocols.

2  Requirements

   Throughout this document, the words that are used to define the
   significance of particular requirements are capitalized.  These
   words are:

      o "MUST"

        This word or the adjective "REQUIRED" means that the
        item is an absolute requirement of this specification.

      o "MUST NOT"

        This phrase means that the item is an absolute prohibition
        of this specification.

      o "SHOULD"

        This word or the adjective "RECOMMENDED" means that there
        may exist valid reasons in particular circumstances to ignore
        this item, but the full implications should be understood and
        the case carefully weighed before choosing a different course.

3 Terminology

   This document uses the following terms:

      o "DHCP client"

        A DHCP client is an Internet host using DHCP to obtain
        configuration parameters such as a network address.

      o "DHCP server"

        A DHCP server is an Internet host that returns configuration
        parameters to DHCP clients.

4  Server Range Option

   Any client that is configured by DHCP[1] may act as an IP packet
   forwarder.  Depending on the network topology, the client may need to
   participate in a routing protocol  conversation with one or more
   neighboring routers.  Part of the input to this conversation is the
   network that is served by the forwarding client.  The attached
   network can be a simple classless range derived from the IP address
   and network mask supplied by the DHCP server to the client.  Using
   the mask to obtain just the network prefix, the client can advertise
   this prefix in the routing protocol conversation.

   The network served by a forwarding client, however, can be composed
   of a series of subnets.  Normally only one of these subnets is
   reflected in the address assigned to a client's interface.  The
   purpose of the Server Range Option is to supply the set of IP
   addresses that the DHCP server is serving to nearby hosts.  From this
   set of addresses, the forwarding client can derive the set of subnets
   to advertise to its router neighbors.  There are two possible
   approaches to supplying the addresses.  Experimentation will
   determine which is most useful.

   The code for this option is 111 and the length is a multiple of 8.

   4.1. Address Range Approach

   The option is built by the server as pairs of starting and ending IP
   addresses, expressing ranges that can represent subnets.  The client
   may have to aggregate ranges to derive a subnet and its mask.

     Code  Len   Start1-address           End1-address
   | 111 |  n  | s1  | s2  | s3  | s4  | e1  | e2  |  e3 | e4

                 Start2-address ...
               | s1  | s2  |  . . .

   4.2 Prefix and Mask Approach

   The option is build by the DHCP server as pairs of subnet prefixes
   (padded with zeroes to 32 bits) and subnet masks.  These are usable
   directly by the client.

     Code  Len   Address-Prefix1          Mask1
   | 111 |  n  | a1  | a2  | a3  | a4  | m1  | m2  |  m3 | m4

                  AddressPrefix2 ...
                | a1  | a2  |  . . .

5  References

   [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
       Bucknell University, October 1993.

   [2] Alexander, S., and R. Droms, "DHCP Options and BOOTP Vendor
       Extensions", RFC 1533, Lachman Technology, Inc., Bucknell
       University, October 1993.

6  Author's Address

   Walter D. Lazear
   MITRE Corporation
   1820 Dolley Madison Blvd.
   McLean  VA  22102

   Phone: 703 883 6515
   EMail: lazear@mitre.org

   This document will expire on August, 1998

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