Classless Routing

As you learned in Hours 4 and 5, the TCP/IP routing system is designed around the concept of a network ID, which is dependent on the address class (A, B, or C) of the IP address. As you also learned in Hour 5, the address class system has some limitations and is sometimes an inefficient method for assigning blocks of addresses to a single provider. Classless Internet Domain Routing (CIDR) offers an alternative method for assigning addresses and determining routes. (See the section titled "Classless Internet Domain Routing" in Hour 5.) The CIDR system specifies a host through an address/mask pair, such as 204.21.128.0/17. The mask number represents the number of address bits associated with the network ID.

The CIDR system offers more efficient routing if the routing protocols support it. CIDR reduces the necessary information that must pass between routers because it lets the routers treat multiple class networks as a single entity. Recent protocols, such as OSPF and BGP4, support classless addressing, but older protocols, such as RIP, do not support CIDR.