Introduction to TCP/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
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 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’s IP address. If there is no gateway in
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Страница 96: ...96 CHAPTER 6 USING THE FIREWALL DIAGNOSTIC TOOLS DUA1611 0AAA02 book Page 96 Thursday August 2 2001 4 01 PM ...
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Страница 178: ...178 CHAPTER 13 TYPES OF ATTACK AND FIREWALL DEFENCES DUA1611 0AAA02 book Page 178 Thursday August 2 2001 4 01 PM ...
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Страница 206: ...206 APPENDIX D TECHNICAL SUPPORT DUA1611 0AAA02 book Page 206 Thursday August 2 2001 4 01 PM ...
Страница 212: ...212 INDEX DUA1611 0AAA02 book Page 212 Thursday August 2 2001 4 01 PM ...
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