Discovering computer networks: hands on in the Open Networking Lab
Discovering computer networks: hands on in the Open Networking Lab

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Discovering computer networks: hands on in the Open Networking Lab

2.5 Fixing a host address, and two special addresses

In this part you will look at some nuts-and-bolts ideas that can be useful when setting up networks or diagnosing problems.

The ‘fixing a host address’ part of the title refers to the assigning of a fixed IP address to a networked device (or host, as it is often called). As you will see in a later session, IP addresses are generally assigned to networked devices automatically through a process called Dynamic Host Configuration Protocol (DHCP), and can change. Sometimes, though, you need a host’s IP address to be fixed. The first section of the video looks at how this can be done to a Microsoft Windows computer.

The remainder of the video looks at two classes of ‘special’ IP address. These addresses have diagnostic use, and are not used as normal IP addresses.

Now watch the video below, which is about 5 minutes long.

Fixing a host address, and two special addresses

Download this video clip.Video player: 15_fixing_a_host_address.mp4
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In this part I want to look at some settings on my personal computer; and I’ll start by going to the Control Panel, Network and Sharing Centre, and then, up here, Change Adapter Settings, and Ethernet – so that’s the port for the wired connection.

Let’s have a look at this: Internet Protocol Version 4 and look at Properties. And this is what I want to bring to your attention: ‘Obtain an IP address automatically’. So that’s what this computer does. It gets its IP address automatically, from the gateway as it happens, and a later session will look at that in more detail.

There is this alternative possibility, here, and sometimes this is handy for diagnostic work where you actually want to give the wired interface an IP number. So I could give it this one [192. 168.2.66], and it knows what the subnet mask is without me typing it in. And I could give it a gateway address []. And if I were to hit ‘OK’ those would be implemented. But I don’t recommend hitting ‘OK’ unless you’re engaged on serious diagnostic work. So I’ll put it back to that [‘Obtain an IP address automatically’], and close that down.

Now, an interesting question that arises is ‘What happens if you boot your computer up so that it can’t get an IP address from anywhere?’ And that’s what I’m going to do now.

I’ve booted up my computer again, and this is what we get. ipconfig, and we get this very characteristic number here: These numbers, beginning 169.254, are what Windows gives you when this automatic IP allocation doesn’t work. It’s called a link-local address, and it’s supposed to have some usefulness, but its main use is for telling you that the IP allocation system isn’t working.

I’m going to demonstrate the loopback address with this PT Anywhere network. There are two computers and this router, but the details of the network aren’t important.

The most important thing for my purposes is that none of the devices has been configured. So none of the devices or interfaces has an IP address or a subnet mask. We could imagine that perhaps we’re about to start configuring this network device by device.

I’m going to start with this computer here. Edit device, Interfaces. I’m giving it an IP address of and the usual subnet mask. And then we do ‘Submit’ and then go to the command prompt. Now I’ll ping this IP address:

Now, you’ll notice that this IP address is very different from the network address that I’ve put on the computer. And as the router hasn’t been configured, and isn’t connected to any other networks anyway, we can be fairly confident that this ping should go nowhere and time itself out. Let’s see. In fact we get successful responses.

When you ping a loopback address, the ping isn’t launched on to the network. It gets as far as being launched, and then gets looped back to the device as though it were a reply from the network. That’s why it’s called loopback.

It does give you a sort of check that the PC has been set up for sending and receiving packets, but of course, it would work even if you’d set up a computer with an IP address that wasn’t compatible with the network. So its value as a check is a bit limited.

Actually, there’s a huge range of loopback addresses. The one I used just now,, is the lowest loopback address.

This one is the highest: And there are the responses.

End transcript
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Now try to answer the questions below.

Activity 5 Test yourself

15 minutes

  • 1. One of the main reasons to give a computer a fixed IP address is to enable it to communicate directly with another device that has a fixed IP address (for example, a printer). Via a direct connection, you can change the device’s IP address or other settings. The direct connection would typically be a length of Ethernet cable between the two devices.

    Suppose you are given a printer that has a fixed IP address of and a subnet mask of Your network uses addresses that begin with 192.168.1 and has the same subnet mask. How do you enable the printer to work on your own network, given that you cannot find a way of doing a factory reset on the printer?

  • Give the Ethernet port of your own computer a fixed IP address that is compatible with the printer’s current address, for example Join the computer to the printer with an Ethernet cable. Use the computer to change the settings of the printer to something compatible with your own network (or to receive IP addresses automatically), and connect the printer to your network. Restore your computer’s Ethernet port to receive IP addresses automatically.

  • 2. A friend rings you to say that their computer is not receiving emails and cannot browse the web. You ask your friend to enter ipconfig at the computer’s command prompt to see whether it has an IP address. Your friend reports that the computer has an IP address, and says, ‘So that cannot be the problem’. Why should you ask what the IP address is?

  • If the automatic allocation of IP addresses has failed, then the computer might have allocated itself a link-local address, which begins with 169.254. This is not a usable address.

  • 3. As a result of 2 above, your friend says, ‘The problem must either be the computer or the network’. What test do you suggest?

  • Ask your friend to ping (or any other loopback address) at the command prompt and see whether there is a reply. If there is a response, then it suggests – but doesn’t prove – that the computer is all right.


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