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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting"

IP Classless Addressing and "Supernetting" Overview, Motivation, Advantages and Disadvantages 1 2 3 4 IP Classless Addressing Block Sizes and "Classful" Network Equivalents

CIDR ("Slash") Notation

Just as subnetting required the use of a subnet mask to show which bits belong to the network ID or subnet ID and which to the host ID, CIDR uses a subnet mask to show where the line is drawn between host ID and network ID. However, for simplicity, under CIDR we don't usually work with 32-bit binary subnet masks. Instead, we use slash notation, more properly called CIDR notation. In this method, we show the size of the network, sometimes called the prefix length, by following an IP address by an integer that tells us how many bits are used for the network ID (prefix).

For example, consider the network specification 184.13.152.0/22. The “22” means this network has 22 bits for the network ID and 10 bits for the host ID. This is equivalent to specifying a network with an address of 184.13.152.0 and a subnet mask of 255.255.252.0, as you can see in Figure 82. This sample network provides a total of 1,022 hosts (2¹⁰ minus 2). The table in the following topic shows all the different possible network sizes that can be configured under CIDR.

IP Classless Addressing and "Supernetting" Overview, Motivation, Advantages and Disadvantages 1 2 3 4 IP Classless Addressing Block Sizes and "Classful" Network Equivalents