A supernet, or classless interdomain routing (CIDR) address, is a group of networks identified by contiguous network addresses. IP service providers can assign customers blocks of contiguous addresses to define supernets as needed. You can use supernetting to address an entire block of class C addresses and avoid using large routing tables to track the addresses.
Each supernet has a unique supernet address that consists of the upper bits shared by all of the addresses in the contiguous block. For example, consider the class C addresses shown in the following figure. By adding the mask 255.255.128.0 to IP address 192.32.128.0, you aggregate the addresses 192.32.128.0 through 192.32.255.255 and 128 class C addresses use a single routing advertisement. In the bottom half of the following figure, you use 192.32.0.0/17 to aggregate the 128 addresses (192.32.0.0/24 to 192.32.127.0/24).
Another example is the block of addresses 192.32.0.0 to 192.32.7.0. The supernet address for this block is 11000000 00100000 00000, with the 21 upper bits shared by the 32-bit addresses.
A complete supernet address consists of an address and mask pair:
The address is the first 32-bit IP address in the contiguous block. In this example, the address is 11000000 00100000 00000000 00000000 (192.32.0.0 in dotted-decimal notation).
The mask is a 32-bit string containing a set bit for each bit position in the supernet part of the address. The mask for the supernet address in this example is 11111111 11111111 11111000 00000000 (255.255.248.0 in dotted-decimal notation).
The complete supernet address in this example is 192.32.0.0/21.
Although classes prohibit using an address mask with the IP address, you can use CIDR to create networks of various sizes using the address mask. With CIDR, the routers outside the network use the addresses.