3Com 3C16111 - SuperStack 3 Firewall Web Site Filter, Руководство пользователя

Страница: 180 / 214

180 C
HAPTER
14: N
ETWORKING
C
ONCEPTS
(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 that contribute to an IP address:
■
IP address itself
■
Subnet mask
■
Default gateway
The following sections discuss each of these components in detail.
IP Address
Just as each household or business requires a unique phone number, a
networked device (such as a computer, printer, file server, or router) must
have a unique IP address. Unlike phone numbers, in IP addressing it is
necessary to always use the entire number when communicating with
other devices.
There are three classes of IP addresses: A, B, and C. Like a main business
phone number that one can call and then be transferred through
interchange numbers to an individual’s extension number, the different
classes of IP addresses provide for varying levels of
interchanges or
subnetworks and extensions or device numbers. The classes are based on
estimated network size:
■
Class A — used for very large networks with hundreds of subnetworks
and thousands of devices. Class A networks use IP addresses between
0.0.0.0
and
127.0.0.0
.
■
Class B — used for medium to large networks with 10–100
subnetworks and hundreds of devices. Class B networks use IP
addresses between
128.0.0.0
and
191.0.0.0
.
■
Class C — used for small to medium networks, usually with only a few
subnetworks and less than 200 devices. Class C networks use IP
addresses between
192.0.0.0
and
223.0.0.0
.
Just as you obtain a phone number from the phone company, 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; if you can estimate the future
growth of the network, this can help you to work out the class and
number of IP addresses you need.
DUA1611-0AAA02.book Page 180 Thursday, August 2, 2001 4:01 PM