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

2.2 Checking the local network

In this part you will use ideas from Section 2.1 to locate and fix problems in a small local network. In the following video, you will see the network in PT Anywhere and the preliminary stages of locating the problem. In the activity following the video, you are asked to complete the investigation and to fix the problem, or problems.

You might need to remind yourself about how to use PT Anywhere by looking again at the Introduction to PT Anywhere video in Session 1.

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

Checking the local network

Download this video clip.Video player: 11_checking_the_local_network.mp4
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You’ve seen that an IP address has four numbers separated by decimal points. On the home networks we’ve looked at, all devices on the network must have the same three numbers at the start of the address. Although our examples had the numbers 192.168.0, other networks might have different numbers.

These three numbers must be the same for all devices on the network. The last number, which I’ve shown as X here, must be different for each device on the network. On our gateway, X was 1; but it doesn’t have to be 1. The important point is that this number must be different for each device on the network.

We saw that the subnet mask shows which numbers of an IP address are for the network, and which for the device. The 255s show the parts of the address that are the network part, and the 0 shows the device part.

If there’s a problem with a network, a good starting point for fixing it is to check that all the devices have IP addresses that satisfy these rules. So we’ll try that with this network.

Let’s suppose someone has reported a problem here. A computer in this network can’t send or receive data, but we don’t know which computer; and we don’t even know what network addresses are being used on this network. A straightforward way to solve the problem would simply be to look at the settings for each device. But I don’t want to do that. I want to track down the problem computer just using ping commands. I’ll start my pinging with the router. Of course, the router itself might be incorrectly configured, but if it were incorrectly configured, more than one computer would be having problems.

To get to the router’s command prompt I just select the router and click on ‘Open console’. Alternatively I can just double-click the router.

Now, what IP number should I ping? I don’t know what addresses are being used on this network. We could try our old friend and see what happens. I’ll do that. Nothing there. If I’m going to work my way through all possible network addresses, this could take a long time.

Instead I’m going to ping this IP number: This broadcasts the ping to everything on the network, whatever its IP address; but I’ll only get replies from devices that are correctly configured. So here are the replies to my ping. It’s pretty clear that this network uses addresses that begin with 192.168.100. Three devices have responded, and they’ve got the numbers 10, 20 and 30 in the fourth place. There are five devices on this network: the router and the four computers. (The switch doesn’t count, because it doesn’t operate at the IP level.) So two addresses are missing. The router hasn’t responded, because that’s the device the ping was sent from, and the sending device doesn’t respond to pings. The other device not represented here is the rogue computer.

So now I’ll go to the command prompt of Mary’s computer and ping A new address has appeared: That must be the router. And the address has disappeared. That must be the address of Mary’s computer, because, as I said just now, a ping isn’t responded to by the computer that sent it.

So we can start to draw up a list of IP addresses and devices on this network. So now on to Advait’s computer. Here I’ll ping again. Mary’s computer,, has reappeared, but the address has disappeared. That must be Advait’s computer.

So your task in the activity below is to fix the network. But you’ll see I’ve added a slight complication.

End transcript
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Activity 2 Try it out

15 minutes

Your task in this activity is to make the PT Anywhere network used in the video [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] work properly. You will see that the network has an additional computer, Winston’s computer, which does not appear in the video, but otherwise the network is identical to the one in the video.

A sensible way to proceed is in four stages:

  1. Find the IP addresses of all working devices.

  2. Identify the non-working device(s). (This should emerge from stage 1.)

  3. Give non-working devices suitable settings.

  4. Check that the network works properly. This is not simply a matter of checking that any devices you have fixed are working properly: it’s also necessary to check that the rest of the network has not been messed up.


My answer can be viewed in the video below, which is about 4 minutes long. In places where I have assigned IP numbers to devices, other IP numbers are possible and would work just as well.

Download this video clip.Video player: 12_answer_to_activity.mp4
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Well, here we are with the network and we now have a new computer here, Winston’s computer. But we’re told that all the rest of the network is as it was before. So I will carry on as I was doing before.

I’ll try Harshi’s computer, and I’m going to ping So the address has disappeared. So that must be Harshi’s computer, and it must be correctly configured. And no new computer has appeared, so Winston’s computer is obviously also incorrectly configured.

So the finger of suspicion points both at Dorothy’s computer as not working and also at Winston’s. If Dorothy’s computer is not correctly configured, then when we ping from there nothing should happen – so we’ll try that. As expected, Dorothy’s computer isn’t communicating with the network.

If we have a look at the settings of Dorothy’s computer, ‘Interfaces’, and we see there that it has a 99 in the third place where it should be 100. But simply changing that to 100 wouldn’t fix the problem, because that would make the IP address identical to Harshi’s. So we’ll make this 40 and ‘Submit’. And we’ll see what happens now: ping So this computer is able to communicate with the other computers and with the router, so that appears to have sorted that problem out.

So let’s have a look at Winston’s. We know that that’s not communicating with the network because it hasn’t appeared in any of the pings we’ve just done. We’ll just check that by pinging from there. No, nothing doing there. So have a look at the settings on that device. And we see it’s got no IP address at all, or subnet mask. So we can give that and subnet mask the usual one255.255.255.0 and ‘Submit’. So what I’m going to do now is go back to the router and try to ping everything from there to check that everything is working. We’re getting replies from 10, 20, 30, 40 and 50, so the network is working as it should.

End transcript
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