Classless Inter-Domain Routing

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Classless Inter-Domain Routing (CIDR) using variable length subnet masks (VLSM) was created to allow for greater flexibility with routed IP networks, to allow for the accelerating expansion of the Internet.

IPv4 IP addresses are 32 bits long and are separated into two parts, the network address and a host address. Historically, IP address 'class' definitions stipulated divisions based on classes with 8, 16, or 24 bit 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.

A variable length subnet mask is a means of allocating IP addressing resources to subnets according to their individual need rather than some general network-wide rule. So therefore the network/host division can occur at any bit boundary in the address. Becuase the normal class distinctions are ignored, the new system was called 'classless'. It came into use in the mid 1990s due to the inefficiences of the classful system.

The standard notation for a CIDR block includes a network number(in 4 octets for IPv4, and a variable length set of up to 8 16-bit hexadecimal fields for IPv6), and a prefix length, in bits, defining the size of the network in question.

For example:

192.168.0.0/24 represents the IPv4 addresses 192.168.0.0 through 192.168.0.255 inclusive, with 192.168.0.255 being the broadcast address for the network.

192.168.0.0/22 represents the IPv4 addresses 192.168.0.0 through 192.168.3.255 inclusive, with 192.168.3.255 being the broadcast address for the network.

2002:C0A8::/48 represents the IPv6 addresses 2002:C0A8:0:0:0:0:0:0 through 2002:C0A8:0:FFFF:FFFF:FFFF:FFFF:FFFF, inclusive.

CIDR/VLSM network addresses are used in many places, including routers and servers both connected and unconnected to the Internet, though an average desktop LAN user generally does not see them in practice.