VLSM was a step up from subnetting because it relayed subnet information through routing protocols. This idea leads directly into this section on CIDR, which is documented in the following RFCs: 1517, 1518, 1519, and 1520. CIDR is an effective method to stem the tide of IP address allocation as well as routing table overflow. Without the implementation of CIDR in 1994 and 1995 in RFC 1817, the Internet would not be functioning today because the routing tables would have been too large for the routers to handle.

The primary requirement for CIDR is the use of routing protocols that support it, such as RIP-2, OSPFv2, and BGP-4. CIDR can be thought of as advanced subnetting. The subnetting mask, previously a number with special significance, ...

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