Network Working Group T. Lemon
Request for Comments: 3442 Nominum, Inc.
Updates: 2132 S. Cheshire
Category: Standards Track Apple Computer, Inc.
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 (C) The Internet Society (2002). All Rights Reserved.
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.
This option obsoletes the Static Route option (option 33) defined in
RFC 2132 .
The IP protocol  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 , ICMP
router discovery [6,9] or using the DHCP Router option, defined in
RFC 2132 .
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
The DHCP protocol as defined in RFC 2131  and the options defined
in RFC 2132  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  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 ) 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  and
STD 5, RFC 950 .
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 .
Lemon, et. al. Standards Track [Page 2]RFC 3442 Classless Static Route Option for DHCPv4 December 2002