A related problem was the rapidly increasing size of the Internet global routing tables. As the number of networks on the Internet increased, so did the routes. A few years back it was forecasted that the global backbone Internet routers were fast approaching their limit on the number of routes they could support. If nothing was done the global routing tables would have reached capacity by mid-1994 and all Internet growth would be halted.
Hierarchical Routing Aggregation To Minimize Routing Table Entries. 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.The scheme is similar to the telephone network. A high level, backbone network node only looks at the area code information and then routes the call to the specific backbone node responsible for that area code. The receiving node then looks at the phone number prefix and routes the call to its subtending network node responsible for that prefix and so on. The backbone network nodes only need routing table entries for area codes, each representing huge blocks of individual telephone numbers, not for every unique telephone number.
There is a maximum number of networks and hosts that can be assigned unique addresses using the IPv4 32-bit long addresses. "classes" of addresses: Class A, Class B and Class C were the most common. Each address had two parts: one part to identify a unique network and the second part to identify a unique host in that network. Another way the old Class A, B, and C addresses were identified was by looking at the first 8 bits of the address and converting it to its decimal equivalent.
Using the old Class A, B, and C addressing scheme the Internet could 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
Restructuring IP Address Assignments
CIDR is a replacement for the old process of assigning Class A, B and C addresses with a generalized network "prefix". Instead of being limited to network identifiers (or "prefixes") of 8, 16 or 24 bits, CIDR currently uses prefixes anywhere from 0 to 32 bits (currently in use 13 to 27 bits). A CIDR address includes the standard 32-bit IP address and also information on how many bits are used for the network prefix. For example, in the CIDR address 66.14.08.48/26, the "/26" indicates the first 26 bits are used to identify the unique network leaving the remaining bits to identify the specific host.
Thus, blocks of addresses can be assigned to networks as small as 2 hosts ("prefix /30") or to those with over 500,000 hosts. This allows for address assignments that much more closely fit an organization's specific needs.