Appendices Page 265
IP Addressing
To become part of an IP network, a network device must have an IP address. An IP address is a
unique number that differen tiat es one d evice fro m anoth er on th e network to av oid conf usion d urin g
communication. To help illustr ate IP addresses, the following sections compar e an IP address to the
telephone numbering system, a system that is used every day.
Like a phone number with its long distance “1” and area code, an IP address contains a set of four
numbers. While we separate phone number components with dashes, for example 1-408-555-
1212, IP address number components are separated by decimal points or dots (called dotted
decimal notation), for example 123.45.67.89. Because computers use a binary number system,
each number in the set must be less than 255.
There are three components of IP addressing:
•IP address
•Subnet mask
•Default gateway

IP Address

Just as each household or bu siness requires a unique phone number, a net worked device (such as
a computer, printer, file se rver, or ro ute r) mus t hav e a un ique IP ad dre ss. Un like p hone n umb ers,
an IP address re quires the entire numb er when communicating with other devices.
There are three classes of IP addres se s: A, B, and C. Like a mai n bu si ne ss pho ne nu m be r that o ne
can call, and then be transferred through interchange numbers to an individual’s extension number,
the different classes of IP ad dr e sse s pr ovi de fo r v a ryin g le v els of “ int e rc han ge s” o r su bnetworks,
and “extensions” or device numbers. The classes are based on estimated network size:
• Clas s A — used for very large networks with hundreds of subnetworks an d thousands of devices.
Class A networks use IP addresses between 0.0.0.0 and 1 27.0.0.0.
• Class B — used for medium to large networks with 10–100 subnetworks and hundreds of de-
vices. Class B networks use IP addresses between 128.0. 0.0 and 191.0.0.0.
• Clas s C — used for small to medium netw orks, usually with only a few subne tworks and less than
250 devices. Class C networks use IP addresses betw een 192.0.0.0 and 223.0.0.0.
Just as one would go to the p hone company for a phone number, there are controlling bodies for IP
addresses. The overall controlling body for IP addresses worldwide is InterNIC. Businesses or
individuals can request one or many IP addresses from InterNIC. It’s a good idea to estimate the
network’s future growth when requesting the class and number of IP addresses requested.