Network Safety Corporation presents...
IP Routing
A Simple Concept
The concept of IP Routing is a simple one, but one that seems to confuse some people.
Lets take a quick look at it.
IP Addresses
An IP Address consists of a 32-bit number that was intended to be unique throughout the
world. This number is usually expressed or written as four decimal numbers separated
by dots. Each number may range from
0 through 255, since each represents an
8-bit value, so that all four together represent the whole 32-bit number.
All of this helps to minimize the wear and tear on our minds caused by mental
conversion between hex and decimal. Here are some examples:
Dotted Hex Binary (argh!)
------------- ------------ ----------------------------------
172.17.33.116 0xAC112174 10101100000100010010000101110100
192.168.2.7 0xC0A80207 11000000101010000000001000000111
10.99.61.25 0x0A633D19 00001010011000110011110100011001
For most people, the "dotted-decimal" notation is the easiest to remember and visuallize.
To use these addresses though, we need to understand...
What is a Network?
In the context of IP Routing, a Network is a virtual place where the IP Addresses of every
workstation and host are exactly the same in the "network part,." differing
only in the "host part." For example, if an organization has a single LAN with a single
Class C Network, say, 192.168.2, then
every device's IP Address will start with the same 24 bits or three octets: 192.168.2. The
remaining octet provides space for 256 unique addresses, some of which are reserved.
In addition, we need the concept of a "netmask." This array of bits contains "one" bits
in each position of the "network part." In this example, where the network part is 24-bits
long, the netmask consists of 24 one-bits and 8 zero-bits. This may be expressed in hex
as 0xffffff00, but it is most often expressed in dotted-decimal as 255.255.255.0.
Clients, hosts and routers use the netmask to decide if two machines are in the same network.
The steps are very simple:
- "AND" the source IP Address with the netmask, which keeps just the "network part.".
- "AND" the destination IP Address with the netmask, to get the "network part.".
- If the two results are identical, the two machines are in the same network.
Computers in "the same network" may communicate with each other directly. They need
no help from anyone. To communicate with a machine in another network, you need...
Routers
Routers are speciallized computers that can exist in more than one network at a time. In
addition, they may have knowledge of other routers. Usually, a small network has a single
router. Because that router is the "only way out" of the network, this router is said to be
the "gateway" to this network. It is permitted to have more than one router in a network,
and your computers usually know what each is for. One may be to reach other parts
of you business or to reach customer sites. One may be to the Internet. That one is
special.
When you do the setup on your workstations and hosts, you have the option to specify a
"default gateway." This is the router to use when you don't know a path to the desired
destination. It is the job of this router to either know how to get to the destination, or to
know a smarter router that may. This smarter router is your router's default gateway. This
is how the entire Internet works, more or less.
It all boils down to these steps:
- If the destination is in your network, send the message directly.
- If the destination is not in your network, and you know a route, send via that router.
- If you haven't a clue as to a route, send by way of your default gateway.
Each computer and each router follows these same rules.
For further reading on this subject, may we recommend "TCP/IP Illustrated, Volume 1" by
W. Richard Stevens. This is published by Addison-Wesley and is ISBN 0-201-63346-9.
This page was last modified on 1 October, 1995.
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