supernetting

supernetting

Combining several IP network addresses into one IP address. Supernetting reduces the number of entries in a routing table and is done in CIDR addressing as well as internal networks.

In the following example, a group of networks with contiguous numbers starting with 172.16.8.0 and ending with 172.16.16.0 are supernetted into the subnet mask of 255.255.224.0. The subnet mask is derived by comparing the binary of the first and last addresses. The last bit location on the right that is the same in both addresses marks the end of the mask. The CIDR notation for this is /19, because there are 19 1 bits in the subnet mask. For example, the IP address 172.16.8.1 would be 172.16.8.1/19. See CIDR.

 IP Network     Binary Representation

 172.16.8.0     10101100.00010000.00001000.00000000
 172.16.16.0    10101100.00010000.00010000.00000000
                                    |
                                    |
 Subnet Mask                        |
                                    |
 255.255.224.0  11111111.11111111.11100000.00000000