11-6
IP Routing—Configuring Static Routes
Overview
CIDR
Classful networks condense more information into fewer bits: a router can
resolve an address into its network and host bits without a 32-bit subnet mask.
However, classful networks do not use IP addresses efficiently. Class C
networks only provide addresses for 254 hosts, while Class B networks
provide addresses for 65,534.
Many organizations need more addresses than a Class C network provides,
but fewer than a Class B network does. Using Class C networks, an organiza-
tion must request another network every time it needs more addresses.
However, if the organization requests a Class B network so that it will have
sufficient addresses, it usually wastes the vast majority of these addresses.
Most IP routers today support Classless Inter-Domain Routing (CIDR), which
allows network administrators to define networks of any size. CIDR typically
uses a prefix length instead of a subnet mask; the number in the prefix is the
number of network bits in the address. For example, a network address with
the subnet mask 255.255.0.0 has a /16 prefix length.
Network administrators can subdivide classful networks into smaller,
variable-length networks by changing the prefix length.
For example, your organization is using the Class B network 172.16.0.0. Your
organization needs at least six subnets, each with at least 500 hosts. With
future expansion, your organization will need ten subnets. You round this
number up to the nearest power of two and decide to divide the network into
sixteen subnets. You calculate that each of the sixteen subnets can hold 4,094
hosts, which more than meets your organization’s requirements.
To subdivide the network, you add one bit to the prefix length for every time
you divide the network in half. For example, half of a /16 network is a /17
network, a fourth of a /16 network is a /18 network, and so forth. Sixteen is
2
4
, so in the scenario outlined above, you would divide the 16-bit network four
times, into sixteen 20-bit subnets:
■ 172.16.0.0 /20 (255.255.240.0)
■ 172.16.16.0 /20
■ 172.16.32.0 /20
■ 172.16.48.0 /20
■ ...
■ 172.16.240.0 /20
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