Classless Inter-Domain Routing (CIDR), also called "Supernetting", is a technique used to reduce the size of routing tables on large networks such as the internet.
A number of contiguous IP subnets can be represented by a single entry with some routing algorithms, allowing for faster routing decisions.
Principally the Border Gateway Protocol (BGP4) and Open Shortest Path First (OSPF) routing protocols support CIDR.
CIDR can also be useful in private class C networks to reduce routing times and generally improve performance. Secondary addressing and multi-site situations are other scenarios where CIDR can be used, or where large class A LAN segments with more than 254 hosts need to be re-addressed.