supernetting

What is supernetting?

The process of combining multiple consecutive network IDs of the same IP address class into a single block. Supernetting, also known as classless interdomain routing (CIDR), is the reverse of subnetting.

How It Works

Supernetting is typically used to conserve class B addresses by combining contiguous groups of class C addresses. The class C addresses must have the same high-order bits, and the subnet mask is shortened by borrowing bits from the network ID and assigning them to the host ID portion to create a custom subnet mask. For example, if a company has 2000 hosts on its TCP/IP network, it can assign IP addresses by

• Using a single class B address. This approach is wasteful.
• Using eight different class C addresses, which can support 8 x 254 = 2032 hosts. This means poorer routing performance because each router requires eight entries in its routing table for each of the eight networks to which frames can be forwarded.
• Using supernetting to collapse a block of eight class C addresses into a single routing table entry. The router must support CIDR for this to work.