3Com OfficeConnect 3C16771, User Manual

Page: 161 / 182

IP Addressing 161
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.
Most large centralized companies have a network manager
in charge of all IP address numbers. Other companies have
a distributed administration scheme that allows the local
network manager to set local IP addresses. In this case, the
local manager gets a sub network or “interchange”
number from the company’s central network manager and
then assigns local IP address numbers.
Subnet Mask
As mentioned in “IP Address” on page 160 , the IP
addressing system allows creation of subnetworks or
interchanges and device numbers or extensions within
those subnetworks. These numbers are created using a
mathematical device called a subnet mask. A subnet mask,
like the IP address, is a set of four numbers in dotted
decimal notation. Subnet masks typically take three forms:
■ 255.0.0.0
■
255.255.0.0
■
255.255.255.0
The number 255 masks out the corresponding number of
the IP address, resulting in IP address numbers that are valid
for the network. For example, an IP address of
123.45.67.89
and a subnet mask of
255.255.255.0
results in a sub network number of
123.45.67.0
and a
device number of
89
. The IP address numbers that are valid
to use are those assigned by InterNIC; this prevents
someone setting up IP addresses that are duplicates of
those at another company.
The subnet mask used for the network typically
corresponds to the class of IP address assigned. If the IP