The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4
RFC 3442

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Network Working Group                                           T. Lemon
Request for Comments: 3442                                 Nominum, Inc.
Updates: 2132                                                S. Cheshire
Category: Standards Track                           Apple Computer, Inc.
                                                                 B. Volz
                                                                Ericsson
                                                           December 2002

                 The Classless Static Route Option for
          Dynamic Host Configuration Protocol (DHCP) version 4

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document defines a new Dynamic Host Configuration Protocol
   (DHCP) option which is passed from the DHCP Server to the DHCP Client
   to configure a list of static routes in the client.  The network
   destinations in these routes are classless - each routing table entry
   includes a subnet mask.

Introduction

   This option obsoletes the Static Route option (option 33) defined in
   RFC 2132 [4].

   The IP protocol [1] uses routers to transmit packets from hosts
   connected to one IP subnet to hosts connected to a different IP
   subnet.  When an IP host (the source host) wishes to transmit a
   packet to another IP host (the destination), it consults its routing
   table to determine the IP address of the router that should be used
   to forward the packet to the destination host.

   The routing table on an IP host can be maintained in a variety of
   ways - using a routing information protocol such as RIP [8], ICMP
   router discovery [6,9] or using the DHCP Router option, defined in
   RFC 2132 [4].

Lemon, et. al.              Standards Track                     [Page 1]
RFC 3442        Classless Static Route Option for DHCPv4   December 2002

   In a network that already provides DHCP service, using DHCP to update
   the routing table on a DHCP client has several virtues.  It is
   efficient, since it makes use of messages that would have been sent
   anyway.  It is convenient - the DHCP server configuration is already
   being maintained, so maintaining routing information, at least on a
   relatively stable network, requires little extra work.  If DHCP
   service is already in use, no additional infrastructure need be
   deployed.

   The DHCP protocol as defined in RFC 2131 [3] and the options defined
   in RFC 2132 [4] only provide a mechanism for installing a default
   route or installing a table of classful routes.  Classful routes are
   routes whose subnet mask is implicit in the subnet number - see
   section 3.2 of STD 5, RFC 791 [1] for details on classful routing.

   Classful routing is no longer in common use, so the DHCP Static Route
   option is no longer useful.  Currently, classless routing [7, 10] is
   the most commonly-deployed form of routing on the Internet.  In
   classless routing, IP addresses consist of a network number (the
   combination of the network number and subnet number described in RFC
   950 [7]) and a host number.

   In classful IP, the network number and host number are derived from
   the IP address using a bitmask whose value is determined by the first
   few bits of the IP address.  In classless IP, the network number and
   host number are derived from the IP address using a separate
   quantity, the subnet mask.  In order to determine the network to
   which a given route applies, an IP host must know both the network
   number AND the subnet mask for that network.

   The Static Routes option (option 33) does not provide a subnet mask
   for each route - it is assumed that the subnet mask is implicit in
   whatever network number is specified in each route entry.  The
   Classless Static Routes option does provide a subnet mask for each
   entry, so that the subnet mask can be other than what would be
   determined using the algorithm specified in STD 5, RFC 791 [1] and
   STD 5, RFC 950 [7].

Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [2].

Lemon, et. al.              Standards Track                     [Page 2]
RFC 3442        Classless Static Route Option for DHCPv4   December 2002

   This document also uses the following terms:

      "DHCP client"

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