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JunosE 11.3.x IP, IPv6, and IGP Configuration Guide
Classless Addressing with CIDR
10
The use of masks can divide networks into subnetworks by extending the network portion
of the address into the host portion. Subnetting increases the number of subnetworks
and reduces the number of hosts.
For example, a network of the form 10.0.0.0 accommodates one physical segment with
about 16 million hosts on it. Figure 4 on page 10 shows how the mask 255.255.0.0. is
applied to network 10.0.0.0. The mask divides the IP address 10.0.0.1 into a network
portion of 10, a subnet portion of 0, and a host portion of 0.1. The mask has borrowed a
portion of the host space and has applied it to the network space. The network space of
the class 10 has increased from a single network 10.0.0.0 to 256 subnetworks, ranging
from 10.0.0.0 to 10.255.0.0. This process decreases the number of hosts per subnet from
16,777,216 to 65,536.
Figure 4: Subnetting
Classless interdomain routing (CIDR) is a system of addressing that improves the scaling
factor of routing in the Internet. CIDR does not use an implicit mask based on the class
of network. In CIDR, an IP network is represented by a prefix, which is an IP address and
an indication of the leftmost contiguous significant bits within this address.
For example, without CIDR, the class C network address 192.56.0.0 would be an illegal
address. With CIDR, the address becomes valid with the notation: 192.56.0.0/16. The /16
indicates that 16 bits of mask are being used (counting from the far left). This would be
similar to an address 198.32.0.0. with a mask of 255.255.0.0.
A network is called a supernet when the prefix boundary contains fewer bits than the
network's natural mask. For example, a class C network 192.56.10.0 has a natural mask
of 255.255.255.0. The representation 192.56.0.0/16 has a shorter mask than the natural
mask (16 is less than 24), so it is a supernet.
Figure 5 on page 11 shows how CIDR can reduce the number of entries globally in Internet
routing tables. A service provider has a group of customers with class C addresses that
begin with 192.56. Despite this relationship, the service provider announces each of the
networks individually into the global Internet routing mesh.
Copyright © 2010, Juniper Networks, Inc.
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