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
