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IntroductiontoTCP/IP 181

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 180, 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

. 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 address is Class A, use a subnet mask of

255.0.0.0

. Class B addresses use a subnet mask of

255.255.0.0

,and

Class C IP addresses use a subnet mask of

255.255.255.0

Default Gateway

A default gateway is like a long distance operator — users can dial the

operator to get assistance connecting to the end party. In complex

networks with many subnetworks, gateways keep traffic from traveling

between different subnetworks unless addressed to travel there. While

this helps to keep overall network traffic more manageable, it also

introduces another level of complexity.

To communicate with a device on another network, the message must go

through a gateway that connects the two networks. Therefore, users

need to know the default gateway’sIPaddress.Ifthereisnogatewayin

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