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Network design
Overview of IP addressing
An IP (v4) address is a 32-bit network address (Layer 3 on the OSI model). An IP address is
usually written in dotted-quad notation. Dotted-quad notation consists of four integer fields that
range from 0 to 255, and are separated by periods.
An IP address consists of a network portion and a host portion. The boundary that separates the
network portion and the host portion of the address is defined by the subnet mask. The subnet
mask is another 32-bit address (again usually written in dotted-quad notation) with a
consecutive string of 1s followed by a consecutive string of 0s when written in binary. All bit
positions of the IP address that are covered by a 1 in the subnet mask are network address bits.
Bit positions that are covered by 0s represent the host address.
Some standard subnet masks have been assigned. For addresses that begin with 0 to 127, the
default subnet mask is 255.0.0.0, in which 8 bits are used for the network, and 24 bits are used
for the host. This is known as a Class A address. For addresses that begin with 128 to 191, the
default subnet mask is 255.255.0.0, in which 16 bits are used for the network, and 16 bits are
used for the host. This is known as a Class B address. Finally, for addresses that begin with 192
to 223, 24 bits are used for the network, and 8 bits are used for the host. This is known as a
Class C address.
In recent years, additional techniques, including Variable Length Subnet Masks (VLSM) and
Classless InterDomain Routing (CIDR), have extended subnetting techniques to make more
efficient use of address space. CIDR introduced the concept of supernets, which is a technique
for aggregating a range of older classful address blocks (for example, Class C) under a single
network mask. VLSM provides a technique for allocating subnets of varying size out of a
classful address block. Prior to this point, once a subnet mask was applied to a network, the
same mask had to be applied to all subnetworks.
Some addresses, defined by RFC 1918, are available for private use. Each address class range
includes one group of addresses. The available addresses are:
Class A: 10.0.0.0 through 10.255.255.255 (mask 255.0.0.0)
Class B: 172.16.0.0 through 172.31.255.255 (mask 255.240.0.0)
Class C: 192.168.0.0 through 192.168.255.255 (mask 255.255.0.0)
These addresses can be allocated by companies and individuals in any way. Be aware,
however, of the following caveats:
These addresses are not routable across the Internet. If an organization that uses RFC
1918 addresses wants to connect to the Internet, that organization must use Network
Address Translation (NAT).
If a company is connecting its network to another company, it must take care that their
RFC 1918 addresses do not overlap. Overlapping address ranges prohibit unimpeded
communication across affected networks.
290 Avaya Application Solutions IP Telephony Deployment Guide
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