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Subnet Addressing
Copyright © 2010, Juniper Networks, Inc.
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.
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 9 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.
NOTE: The router supports a 31-bit mask on point-to-point links. This means
that the IP address 1.2.3.4 255.255.255.254 is valid. A point-to-point link in
which only one end supports the use of 31-bit prefixes may not operate
correctly.
Figure 3: Basic Network Masking
Classes A, B, and C have the following natural masks, which define the network and host
portions of each class:
Class A natural mask 255.0.0.
Class B natural mask 255.255.0.0
Class C natural mask 255.255.255.0
Chapter 1: Configuring IP
9
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