Just as the Internet was growing at an exponential rate in the early 1990s, so were the size of the routing tables that were maintained by Internet routers under classful IP addressing. For this reason, the IETF introduced CIDR in RFC 1517 in 1993.
CIDR replaced the classful network assignments and address classes (A, B, and C) became obsolete. Using CIDR, the network address is no longer determined by the value of the first octet. Instead, the network portion of the address is determined by the subnet mask, also known as the network prefix, or prefix length (i.e., /8, /19, etc.).
ISPs are no longer limited to a /8, /16, or /24 subnet mask. They can now more efficiently allocate address space using any prefix length, starting with /8 and larger (i.e., /8, /9, /10, etc.). The figure shows how blocks of IP addresses can be assigned to a network based on the requirements of the customer, ranging from a few hosts to hundreds or thousands of hosts.
CIDR also reduces the size of routing tables and manages the IPv4 address space more efficiently using:
- Route summarization - Also known as prefix aggregation, routes are summarized into a single route to help reduce the size of routing tables. For instance, one summary static route can replace several specific static route statements.
- Supernetting - Occurs when the route summarization mask is a smaller value than the default traditional classful mask.
Note: A supernet is always a route summary, but a route summary is not always a supernet.