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IP Routing Primer: Part One

November 6, 2000
 by Peter Morrissey

Table of Contents

 

In many ways, the task of a router is very simple. It must look at the destination address of an IP packet and then forward the packet to another router or to a directly connected host. However, if you've ever attempted to get two devices to communicate on an IP network, you know that it doesn't always work.

In this Network Design Manual article we'll show you the steps involved in getting a packet from Point A to Point B on an IP network. We'll talk about the settings that are necessary on a host to communicate on a routed network and what they really mean. Once you understand this, you'll be able to ask the right questions when things don't go as expected and even do a little bit of your own troubleshooting.

Anatomy of an IP Address

When one host has data to send to another host, it sends it to its local IP process, which builds an IP packet for transmission on the network. The packet has a header, which includes the destination IP address as well as the source address of the local host. The destination and source IP addresses stay with the data until it arrives at the host so that all the routers it traverses know in what direction to send it. The source address provides the destination host with an address to which it can send its response.

To understand how this happens requires a deeper knowledge of the IP address. It's obvious that an IP address consists of four separate numbers, separated by dots. Each number represents one byte worth of data, limiting it to a range of 256 possible values, commonly represented as decimal values from 0 to 255 or binary values from 00000000-11111111.

One thing that is not so obvious is that each IP address is always divided into two sections. The first section always delineates the network address, or subnet. This is the part routers are primarily concerned with. The second section, combined with the first section forms a unique node address. The subnet mask is configured to tell the host or client which part is the network address and which part is the node address.

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