TCP/IP: The Anatomy of the Most Important Protocol on the Planet

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Introduction

This is the second part in our popular Networking Guide series explains the important bits that you need to know about the world's most popular networking protocol (TCP/IP).

In learning about home and small office networks, we need to understand the key components of TCP/IP and how to apply these principles to our own needs.

We aim to explain these essential networking terms to you in plain, easy to understand language so that you can take a step closer to becoming the networking "guru" in your home or small office.

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Contents

• Introduction
• Prerequisite Knowledge: Before You Read This Article..
• Objective
• Jargon Primer: TCP/IP - What Does It Mean?
• An IP Address
• Host & Network Identifiers
• Subnet Mask
• Default Gateway
• Public & Private Networks
• Putting It All Together: An Example of a Small Network
• Summary

Prerequisite Knowledge: Before You Read This Article..

We are keen that our articles help your understanding. However, as we're now getting into some areas of reasonable technical complexity, it's important that you have the basic knowledge mastered before proceeding any further!

As a very minimum, you should have:

• Read our previous article Basic Principles of Networking
• Have a basic understanding of what a "protocol" is and how / why it is used

If you are very new to networking, we would encourage you to stop reading here and go back to read and understand these important basic principles first!

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Objective

On reading this article you should understand the following concepts and their significance to setting up a home or small office network:

• IP Address
• IP Subnet Mask
• Default Gateway

The value to us in understanding these concepts is that we will easily be able to plan, create & maintain our own home or small office networks by ourselves (hence saving time & money compared to having to call in an external "expert").

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Jargon Primer: TCP/IP - What Does It Mean?

TCP/IP stands for "Transmission Control Protocol / Internet(working) Protocol". It is a description of a networking protocol that has evolved over several decades and is now the most widely used networking protocol. It is also the protocol that forms the "backbone" of the way the internet works.

For an explanation of what a "protocol" is, please see our previous part of this series.

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An IP Address

Diagram 1 shows an example of what a typical IP address looks like. Important things about an IP address are:

• There are EXACTLY(no more, no less) four numbers separated by a "period"("Full Stop" in GB English).
• Each number MUST be equal or greater to 0 and no more than 255.
• We know that this IP address is on a "Private" network. (More on this later)

The example in Diagram 1 shows us a set of numbers that are easily read by humans. From your basic technology / science lessons at school, you know that computers and humans think in different ways.

• Computers: "Think" & communicate in ones and zeros (1 & 0)
• Humans: Think & communicate in diverse ways such as different cultural languages. Our number system is based on the concept of ten(10)

For the purposes of this article, we'll stick to the human readable forms of expressing IP information. However, you need to be aware that, under the hood of the TCP/IP engine, computers are using and "seeing" IP addresses as a sequence of ones and zeros. Understanding the ones and zeros is only necessary if you get into complex "subnetting" (more on this later).

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Host & Network Identifiers

Networks are made up of two key things:

Computers / Devices

Referred to as HOSTS in network "speak".

Networks

Simply called NETWORKS. A NETWORK is defined as a bunch of devices hooked up together to form a network.

Diagram 2 shows a very simple network of devices connected together. The important things that you need to understand at this point are:

• There are HOSTS and NETWORKS
• A HOST exists in a NETWORK
• A HOST can be any connected device (just like the diagram shows!)

In Diagram 2, we've shown the simplified model of a network. Now let's see how this applies to a real life TCP/IP network (e.g. one that you might set up at home or in the office).

Diagram 3 now adds some more information relating to actual TCP/IP.

You will notice that Network "N" is designated as 192.168.1.x. Sometimes, you'll see this expressed with the "x" replaced by a zero(0):- It means the same thing.

So, we are getting quite close to understanding the basics about TCT/IP now! We should now understand how HOSTS are related to each other and to a network.

Having a network like this is great and means that all of our devices can talk to each other at home or in the office. However, the real advantages of networking come when we hook networks together. For example, at home or in the office, we might want to hook our network up to the Internet (it's just another network!) so that all of our computers can send/receive email. Let's look at this in the next diagram.

Diagram 4 shows nearly the same thing as diagram 3 in terms of an individual network. However, the main thing it shows is how multiple networks can be hooked up together.

If you're sticking with this stuff so far, you might have noticed that we've sneakily thrown another device into the diagram. Yes, we admit it! It's the ROUTER that now is now included in both networks. A router is simply another device connected to a network. It is responsible for "routing" any TCP/IP traffic to and from other networks. Every network that needs to "talk" to another network needs some form of router.

Now that we've come to the point where we understand what HOSTS and NETWORKS actually are, let's relate this back to TCP/IP designations. Very simply put:

A HOST Identifier

Uniquely identifies a device on a network. For example, if your IP address on your network is 192.168.1.1, your HOST ID might be "1"

A NETWORK Identifier

Uniquely identifies your network. For example, if your IP address on your network is 192.168.1.1, your NETWORK ID might be "192.168.1.x"

Don't worry if you don't yet fully understand Network and Host identifiers. It will help you to read the next section (Subnet Mask).

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Subnet Mask

So far you will have learnt that there are HOSTS and NETWORKS and that a HOST exists in a NETWORK. Now we are going to put the final piece of the puzzle in place and talk about the Subnet Mask.

Diagram 5 shows us an exploded view of a HOST on a NETWORK. The subnet mask simply defines which sections of an IP address are the NETWORK ID and which section(s) is/are the HOST ID. We've taken an example of a subnet of 255.255.255.0. You can use this example to set up your own network.

So, we should now be at a point where we understand that a subnet mask DEFINES the network to the HOSTS. For example, if HOSTS "A" and "B" are both connected together, and both have the same subnet mask defined they are deemed to be on the same network as each other.

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Default Gateway

In the previous section, we discussed how a subnet mask DEFINES a network (by Network ID) to a HOST.

What happens when HOSTS have different subnet masks? Well, you guessed right if you were thinking that HOSTS with different subnet masks are considered to be on different networks!

Look back at diagram 4. For HOSTS in different networks to be able to connect and communicate, they have to communicate via a series of ROUTERS.

The Default Gateway is simply the address of the router on your network that is responsible for communicating with the outside networks.

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Public & Private Networks

Public Network (Sometimes called a WAN)

The Internet is a public network. Every IP address on The Internet must be unique.(i.e. globally unique).

Private Network (Sometimes called LAN)

A private network could be like the one that you have at home or in the office. It may (optionally) be connected to a public network. IP addresses in a private network need to be unique but do not need to be globally unique.

It is important that you follow international standards when setting up private networks. You must use IP addresses that are in certain ranges so as not to duplicate ones already in use on public networks. This is particularly important where your private network is connected up to the internet:- It won't harm The Internet but it will make your private network non-functional.

The standards (as defined by Internet Assigned Numbers Authority (IANA)) for private network addresses are:

• 10.0.0.0 to 10.255.255.255
• 172.16.0.0 to 172.31.255.255
• 192.168.0.0 to 192.168.255.255
• Also, IP addresses in the range of 169.254.0.0 to 169.254.255.255 are reserved for Automatic Private IP Addressing (such as in Windows XP)

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Putting It All Together: An Example of a Small Network

Diagram 6 shows a typical configuration of a home or small office network that is connected to the Internet by a Broadband Router.

You should be able to easily understand this example now. Let's list some of the important features:

• All HOSTS are on the same network. We know this because they all have the same subnet mask of 255.255.255.0
• Each HOST has a unique IP address in its own network
• The maximum number of hosts that we can have on this network is 254
• Network traffic destined for other networks (i.e. The Internet in this example) is sent to the DEFAULT GATEWAY which is the router at 192.168.1.254 in this example

You can use this example to install and configure your own network and hook it up to a broadband router.

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Summary

We've covered a lot of ground in this article. Some of it has been (unavoidably) quite technical but we've tried to make it as easy to digest as possible.

If you read the previous part of this series, you will remember how a network is like a telephone system (?). Hopefully by now, you should see the similarities and we hope that the comparison of the telephone system has made networking easier to understand.

Here are the key points that we have covered in this article:

• TCP/IP: what is it?
• IP Addresses
• Hosts and Networks
• Subnets
• Default Gateways & Routers

We wrapped it all up with an example of a network that you could implement at home or in a small office environment.

Having completed this article, you now have the fundamental knowledge to go and design and install a simple network. However, don't get too complacent quite yet! There is still lots to learn such as auto address assignment, Directory Services, Security, Wireless Networking and a whole bunch of other concepts. Keep a look out for the next in the series from our Networking Guides series.

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