IP Supernetting

Supernetting is the idea of combining two or more blocks of IP addresses that together compose a continuous range of addresses (no missing addresses in the middle). You create a supernet when you have a need to place more hosts on a single network than currently will work in a classful configuration.

The term supernetting stems from the idea that at one time there were 'classes' of IP addresses and that certain classes of addresses were a certain size. By adding two sets of addresses of one size using supernetting, you created a larger set of addresses--a super subnet that contained both of the smaller classful subnets.

Classful addressing identifies a class C addresses as being in the range of 192.0.0.0 through 223.255.255.255. A single class C block such as 192.168.1.0 - 192.168.1.255 has at most, 254 addresses. If you have more than 254 computers that need to be on the same network (to get Windows Domain Browsing working for instance). then you need to create a supernet.

You may recall that every IP address contains a host portion that identifies the individual host and a network portion that identifies which network the host is attached to. Under the classful addressing scheme, no masks are needed. The address itself describes which part of the IP address is the network portion and which part of the address is the host portion.

The idea of supernetting is the idea of borrowing one or more bits from the network portion of the address and using it to create a larger set of hosts.