RFC 932 (rfc932) - Page 2 of 4
Subnetwork addressing scheme
Alternative Format: Original Text Document
RFC 932 January 1985 A Subnetwork Addressing Scheme The gateways and hosts attached to this subnetted network use these addresses as ordinary Class C addresses. Thus, no modification to any host software is required for hosts attached to a subnetwork. For gateways not directly attached to the subnetted network, it is an unacceptable burden to separately store the routing information to each of the subnets. The goal of any subnet addressing scheme is to provide a strategy by which distant gateways can store routing information for the network as a whole. In this scheme, since the first two bytes of the address is the same for every subnet in the network, those first two bytes can be stored and manipulated as if they are a single Class B address by a distant gateway. These addresses, which can be used either as a Class B or Class C address as appropriate, have been informally called Class "B 1/2" addresses. In more detail, a gateway would treat Class C addresses as follows under the scheme. First, test to see whether the high order bit of the address is on. If not, the address is an ordinary Class C address and should be treated as such. If the bit is on, this Class C address identifies a subnet of a network. Test to see if this gateway is attached to that network. If so, treat the address as an ordinary Class C address. If the gateway is not attached to the network containing that subnetwork, discard the third byte of the Class C address and treat the resulting two bytes as a Class B address. Note that there can be no conflict between this two-byte pattern and an ordinary Class B address, because the first bits of this address are not those of a valid Class B address, but rather those of a Class C address. OPTIMIZATIONS If a network grows to more than 256 subnetworks, it will be necessary to design two distinct blocks of special Class C addresses, and to view this aggregate as two separate networks. However, the gateways of these two networks can, by proper design, run a joint routing algorithm which maintains optimal routes between the two halves, even if they are connected together by a number of gateways. Indeed, in general it is possible for gateways that are not directly attached to a subnetworked network to be specially programmed to remember the individual Class C addresses, if doing so provides greatly improved network efficiency in some particular case. It was stated earlier that no modification to the host software is necessary to implement this scheme. There is one case in which a Clark
