Supernetting combines multiple class C networks to appear as a virtual class B network.

Class B networks are now very scarce. A company can buy a block of eight class C address that all share the same high-order bits from InterNIC. To appear as a class B, the subnet mask is configured so there are less networks and more hosts by letting some of the network bits act as host bits.

When routing decisions are to be performed, the subnet mask passes the bits for marking which subnet is used. This allows a group of class C networks to appear as though they are all on the same subnet.

Supernetting is basically the opposite of subnetting.