RFC 3442 (rfc3442) - Page 2 of 9
The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4
Alternative Format: Original Text Document
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