Planning Supernetting and Classless Interdomain Routing (CIDR)

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Similar to the way that subnetting allows you to divide class-based networks into smaller subnets by "borrowing" bits from the host part of the address, supernetting allows you to combine contiguous subnets into larger supernets by "borrowing" bits from the network part of the address. For example, rather than allocate a Class B network ID to an organization that has 2,000 hosts, the Internet Assigned Numbers Authority (IANA) might allocate a range of eight Class C network IDs. Each Class C network ID accommodates 254 hosts, for a total of 2,032 host IDs.

Although this technique helps conserve Class B network IDs, it creates a new problem. Using conventional routing techniques, the routers on the Internet must, in this example, have eight Class C network ID entries in their routing tables to route IP packets to the organization. To prevent Internet routers from becoming overwhelmed with routes, a technique called Classless Interdomain Routing (CIDR), which the Internet uses to summarize routes, collapses multiple network ID entries into a single entry. In this example, CIDR collapses the network IDs that correspond to the eight Class C network IDs allocated to that organization into one entry.

A supernetted subnet mask conveys the starting network ID and the number of Class C network IDs allocated. The following tables demonstrate how eight Class C network IDs are allocated. Table 1.1 indicates the contiguous allocation of eight Class C network IDs, starting with network ID 220.78.168.0. Note that the first 21 bits (underlined) are the same for the starting network ID and the ending network ID. The last 3 bits of the third octet, which are borrowed from the network ID, range from 000 through 111. In decimal notation, the range is 0 through 7, or 8 total contiguous subnets, which are combined into one supernet.

Table 1.1   Supernetted Block of Addresses

	Network ID (decimal)	Network ID (Binary)
Starting Network ID	220.78.168.0	11011100 01001110 10101000 00000000
Ending Network ID	220.78.175.0	11011100 01001110 10101111 00000000

A block of supernetted addresses, such as those in Table 1.2, is known as a CIDR block. Table 1.2 indicates the single CIDR entry that appears in the routing table. This entry represents all eight Class C network IDs that are allocated to the example organization.

Table 1.2   CIDR Routing Table Entry

Network ID	Subnet Mask	Subnet Mask (Binary)
220.78.168.0	255.255.248.0	11111111 11111111 11111000 0000000

In network prefix length notation, the CIDR block is 220.78.168.0/21.

RIP v2, OSPF, and BGP4, which can exchange routing information in the form of [Network ID, Network Mask] pairs, support CIDR.