Updated Oct 22 2002

Designing a simple TCP/IP Network

Note: words that are italicized are explained in the glossary at the bottom of this document.

Note : This document assumes you are not running TCP/IP on your LAN and that you have no Internet connection through your LAN. It also assumes that you will not be connecting your LAN to the Internet using the settings in the provided example. This is for a simple singlesegment LAN.

Basics - Each host on a TCP/IP network has a unique IP address. When sending information between two hosts or a computer and a network printer, you are sending it to the IP address of the intended host. (It?s actually more complicated than that but for what we need to know this is true enough.)

An IP address is broken up into two parts. A network address and a host address. Each host address is unique and defines each host on that network. The network address defines which network you are on. All hosts on the same segment MUST have the same network address.

To find out where the network portion ends and the host portion begins in an IP address, we use a subnet mask. The subnet mask can be thought of as a filter. Its purpose is to separate and define the network portion of the IP address and what is left is the host portion. The subnet mask will be defined by the class of the IP address.

An IP address is four octets (bytes) long, usually represented in decimal form from 000 to 255. A basic IP address would look like this : 192.168.17.6 The subnet mask for this example is : 255.255.255.0

therefore the network portion for this example is : 192.168.17 and the host portion is 6
 
IP Address
1
9
2
.
1
6
8
.
0
1
7
.
0
0
6
Subnet mask
2
5
5
.
2
5
5
.
2
5
5
.
0
0
0
Network Address
1
9
2
.
1
6
8
.
0
1
7
.
-
-
-
Host Address
-
-
-
.
-
-
-
.
-
-
-
.
0
0
6

This leaves addressing for 256 hosts but host numbers 0 and 255 are reserved and can not be used. This then leaves 254 host addresses.
Our first host IP address would be 192.168.17.1
Our second host IP address would be 192.168.17.2
And so on.

The subnet mask is the same on all hosts 255.255.255.0

Several blocks of addresses have been reserved for use with networks that will not be connected to the Internet. These addresses include:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255

There are some special reserved addresses that can not be used when designing a network. These include:
0.0.0.0 127.0.0.0 - 127.255.255.255
128.0.0.0 191.255.0.0
192.0.0.0 240.0.0.0 - 255.255.255.255

Glossary:
Host(s) : any device that is directly connected to the network. Example. Computer, network printer, router.
Segment : a collection of hosts connected together without going through a bridge or router.
Class : when using classful IP addressing the subnet mask is defined by the IP address.

Class
First octet
Subnet mask
Class A
0-127
255.0.0.0
Class B
128-191
255.255.0.0
Class C
192-223
255.255.255.0
Class D & E
224-239,240-255
D-multicasting, E-experimental

If you want to learn more about IP addressing try:


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