Classless Interdomain Routing (CIDR) for IPv4
A version of this page is also available for
4/8/2010
Also known as supernetting, classless inter-domain routing (CIDR) can be used in IPv4 to consolidate several class C network addresses into one logical network. CIDR is described in RFCs 1518 and 1519. To use supernetting, the IP network addresses that are to be combined must share the same high-order bits, and the subnet mask is shortened to take bits away from the network portion of the address and add them to the host portion. For example, the class C network addresses 199.199.5.0, 199.199.6.0, and 199.199.7.0 can be combined by using a subnet mask of 255.255.252.0 for each.
NET 199.199.5 (1100 0111.1100 0111.0000 0101.0000 0000)
NET 199.199.6 (1100 0111.1100 0111.0000 0110.0000 0000)
NET 199.199.7 (1100 0111.1100 0111.0000 0111.0000 0000)
MASK 255.255.252.0 (1111 1111.1111 1111.1111 1100.0000 0000)
When routing decisions are made, only the bits covered by the subnet mask are used, thus making these addresses all appear to be part of the same network for routing purposes. Any routers in use must also support CIDR and may require special configuration.
Note
CIDR is specific to IPv4, and is not supported in IPv6.
For information about managing subnet multicast membership for IPv6, see Multicast Listener Discovery (MLD).