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look here '''Classless Inter-Domain Routing''' ('''CIDR''') using '''variable length subnet masks''' ('''VLSM''') was created to allow for greater flexibility with routed [[internet protocol|IP]] networks, to allow for the accelerating expansion of the [[Internet]]. CIDR is defined in RFC 1519.
[[IPv4]] [[IP address]]es are 32 bits long and are separated into two parts, the network address and a host address. Historically, IP address '[[classful network|class]]' definitions stipulated divisions based on classes with 8, 16, or 24 bit network address fields (known as classes A, B and C respectively). The class, and hence the length of the subnet mask could always be determined from the most significant three bits of the IP address. Without any other way of specifying the length of a subnet mask, [[routing|routing protocols]] necessarily used the class of the IP address specified in route advertisements to determine the size of the routing prefixes to be set up in the [[routing table]]s.
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