RFC 3442 (rfc3442) - Page 2 of 9

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

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].








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