| Routers use a form of IPv4 addressing
called classless interdomain routing (CIDR) (pronounced
"cider") that ignores class. 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 bitmask to determine the network and host portions of an
address, which are no longer restricted to using an entire octet.
First introduced in 1993 by RFC 1517,
1518, 1519, and 1520, and later deployed in 1994, CIDR dramatically
improves IPv4's scalability and efficiency by providing the
following:
- The replacement of classful
addressing with a more flexible and less wasteful classless
scheme
- Enhanced route aggregation, also
known as supernetting
The following sections describe route
aggregation, supernetting, and address allocation in more detail.
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