Classless Inter-Domain Routing (CIDR), sometimes known as supernetting, is a new addressing scheme for the Internet which allows for more efficient allocation of IP addresses than the old Class A, B, and C address scheme. The Internet is currently a mixture of both "CIDR-ized" addresses and old Class A, B and C addresses. Almost all new routers support CIDR and the Internet authorities strongly encourage all users to implement the CIDR addressing scheme.
The original Internet Protocol defines IP addresses in the following classes:
| Address Class | # Network Bits | # Hosts Bits | Decimal AddressRange |
| Class A | 8 bits | 24 bits | 1-126 |
| Class B | 16 bits | 16 bits | 128-191 |
| Class C | 24 bits | 8 bits | 192-223 |
This class addressing scheme could only support the following:
- 126 Class A networks that could include up to 16,777,214 hosts each
- Plus 65,000 Class B networks that could include up to 65,534 hosts each
- Plus over 2 million Class C networks that could include up to 254 hosts each
CIDR addresses reduce the size of routing tables and make more IP addresses available within organizations. Using CIDR, each IP address has a network prefix that identifies either an aggregation of network gateways or an individual gateway. The length of the network prefix is also specified as part of the IP address and varies depending on the number of bits that are needed, instead of an arbitrary class assignment.
| CIDR Block Prefix | # Equivalent Class C | # of Host Addresses |
| /27 | 1/8th of a Class C | 32 hosts |
| /26 | 1/4th of a Class C | 64 hosts |
| /25 | 1/2 of a Class C | 128 hosts |
| /24 | 1 Class C | 256 hosts |
| /23 | 2 Class C | 512 hosts |
| /22 | 4 Class C | 1,024 hosts |
| /21 | 8 Class C | 2,048 hosts |
| /20 | 16 Class C | 4,096 hosts |
| /19 | 32 Class C | 8,192 hosts |
| /18 | 64 Class C | 16,384 hosts |
| /17 | 128 Class C | 32,768 hosts |
| /16 | 256 Class C | 65,536 hosts |
| (= 1 Class B) | ||
| /15 | 512 Class C | 131,072 hosts |
| /14 | 1,024 Class C | 262,144 hosts |
| /13 | 2,048 Class C | 524,288 hosts |
A specific CIDR network address looks like this:
192.30.250.00/21
The "192.30.250.00" is the network address itself and the "21" says that the first 21 bits are the network part of the address, leaving the last 11 bits for specific host addresses. The CIDR addressing scheme also enables "route aggregation" in which a single high-level route entry can represent many lower-level routes in the global routing tables. This aggregation of networks in a single address is sometimes referred to as a supernet.
Currently, big blocks of addresses are assigned to the large Internet Service Providers (ISPs) who then re-allocate portions of their address blocks to their customers. These customers, who may be smaller ISPs themselves, in turn re-allocate portions of their address block to their users and/or customers.
CIDR is supported by newer routing protocols such as Border Gateway Protocol(BGP) and the Open Shortest Path first (OSPF) protocol.
Class A, B and C address allocation:
CIDR address allocation:
Related Terms: IP address, BGP, OSPF