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| 2.2 |  | IP Addressing Crisis and Solutions |
| 2.2.2 |  | Classless Interdomain Routing (CIDR) |

Routers use a form of IPv4 addressing called Classless Interdomain Routing (CIDR) that ignores class.

> Note: CIDR is pronounced cider. In a classful system, a router determines the class of an address and then identifies the network and host octets based on that class. With CIDR, a router uses a bit mask to determine the network and host portions of an address, which are no longer restricted to using an entire octet.

CIDR was first introduced in 1993 by RFC 1517, 1518, 1519, and 1520, and later deployed in 1994. CIDR dramatically improves the scalability and efficiency of IPv4 by providing the following:

- Replacement of classful addressing with a more flexible and less wasteful classless scheme.
- Enhanced route aggregation, also known as supernetting or summarization.
- Supernetting, which is the combination of contiguous network addresses into a new address defined by the subnet mask.

The following sections describe route aggregation, supernetting, and address allocation in more detail.