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Subnetwork Mask Format Options
Most commands allow you to specify IPv4 subnetwork masks in one of two ways:
dotted decimal or prefix length notation.
NOTE: Protocol commands that use a reverse mask format (for example, RIP) cannot
use the prefix notation format. Use the CLI help to verify if a command supports the
/N prefix notation.
Dotted decimal notation expresses IP addresses and masks in dotted quads - four
octets separated by dots (A.B.C.D). In this format, each octet in the address or mask
is represented as a decimal number and the dots are used as octet separators.
For example, an IP address and subnetwork mask in dotted decimal notation would
appear as follows:
10.10.24.6 255.255.0.0
Prefix length notation (often called network prefix format) allows for more efficient
allocation of IP addresses than the old Class A, B, and C address scheme. The prefix
length is the number of leftmost contiguous bits equal to 1 in the subnetwork mask.
This format appears immediately following the dotted decimal IP address using a /N
format.
NOTE: You can issue the network prefix with or without a space between the IP
address and the network prefix (/N).
For example, the same IP address and subnetwork mask mentioned above would
appear as follows using /N format:
10.10.24.6/16
or
10.10.24.6 /16
The following sections describe each subnetwork mask addressing method in more
detail.
Subnet Addressing
A subnet is a subset of a class A, B, or C network. Subnets cannot be used with class
D (multicast) addresses.
A network mask is used to separate the network information from the host
information about an IP address. Figure 3 on page 10 shows the network mask
255.0.0.0 applied to network 10.0.0.0. The mask in binary notation is a series of 1s
followed by a series of contiguous 0s. The 1s represent the network number; the 0s
represent the host number. The sample address splits the IP address 10.0.0.1 into
a network portion of 10 and a host portion of 0.0.1.
Chapter 1: Configuring IP
9
IP Addressing
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