Classless Inter-Domain Routing (CIDR)
CIDR, described in RFCs 1518 and 1519, removes the concept of address classes from the IP address assignment and management process. In place of predefined, well-known boundaries, CIDR allocates addresses defined by a network prefix, which makes more efficient use of available space. The network prefix defines the portion of the address that is fixed. For example, an assignment from an ISP to a corporate client might be expressed as 157.60.1.128/25. In this prefix, the first 25 bits are fixed and the last 7 bits can be used for address assignment. This would result in a 128-address block for local use, with the upper 25 bits being the network identifier part of the address. A legacy, class-full prefix would be expressed as w.0.0.0/8, w.x.0.0/16, or w.x.y.0/24. As these are reclaimed, they will be reallocated using classless CIDR techniques.
Given the installed base of classful systems, the initial implementation of CIDR was to summarize portions of the Class C address space. This process was called supernetting. Supernetting can be used to consolidate several network IDs into one prefix. For example, the network IDs 131.107.4.0, 131.107.5.0, 131.107.6.0, and 131.107.7.0 can be summarized with the network ID 131.107.4.0 with a subnet mask of 255.255.252.0 (131.107.4.0/22). For example:
NET 131.107.4.0 (1100 0111.1100 0111.0000 0100.0000 0000)
NET 131.107.5.0 (1100 0111.1100 0111.0000 0101.0000 0000)
NET 131.107.6.0 (1100 0111.1100 0111.0000 0110.0000 0000)
NET 131.107.7.0 (1100 0111.1100 0111.0000 0111.0000 0000)
MASK 255.255.252.0 (1111 1111.1111 1111.1111 1100.0000 0000)
All four of the network IDs share the same high-order 22 bits.
When routing decisions are made, only the bits covered by the subnet mask are used, thus making all these addresses appear to be part of the same network for routing purposes. Any routers in use must also support CIDR and may require special configuration.
Windows Server 2003 TCP/IP includes support for the all-0's and all-1's subnets as described in RFC 1878.
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