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JunosE 11.3.x IP, IPv6, and IGP Configuration Guide
Subnetwork Mask Format Options
8
Four types of IP classes lend themselves to different network configurations, depending
on the desired ratio of networks to hosts. Figure 2 on page 8 shows the format of IP
address classes.
Figure 2: IP Address Classes
Class A—The leading bit is set to 0, a 7-bit number, and a 24-bit local host address.
Up to 125 class A networks can be defined, with up to 16,777,214 hosts per network.
Class B—The two highest-order bits are set to 1 and 0, a 14-bit network number, and
a 16-bit local host address. Up to 16,382 class B networks can be defined, with up to
65,534 hosts per network.
Class C—The three leading bits are set to 1, 1, and 0, a 21-bit network number, and an
8-bit local host address. Up to 2,097,152 class C networks can be defined, with up to
254 hosts per network.
Class D—The four highest-order bits are set to 1, 1, 1, and 0. Class D is used as a multicast
address.
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
Copyright © 2010, Juniper Networks, Inc.
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