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IP "Supernetting": Classless Inter-Domain Routing (CIDR) Hierarchical Addressing and Notation
(Page 2 of 4)
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).
 Key Concept: Since there are no address classes in CIDR, one cannot tell the size of the network ID of an address from the address alone. In CIDR, the length of the prefix (network ID) is indicated by placing it following a slash after the address. This is called CIDR notation or slash notation.
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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.
Figure 82: CIDR (“Slash”) Notation and Its Subnet Mask Equivalent A classless network is normally specified in CIDR or “slash” notation, such as this example: 184.13.152.0/22. Here, the “/22” means the first 22 bits of the address are the network ID. The equivalent subnet mask can be calculated by creating a 32-bit number with 22 ones followed by 10 zeroes.
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 Note: You may recall my mentioning that under “classful” subnetting, the bits used for the subnet ID did not need to be contiguous. Even though to avoid confusion this was rarely if ever used, non-contiguous subnet ID bits were possible. Under CIDR, the requirement for contiguous subnet ID bits has been made official—you could not use slash notation otherwise.
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