2.5 Fixing a host address, and two special addresses

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 [192.168.2.254]. 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: 169.254.34.148. 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 192.168.1.10 and the usual subnet mask. And then we do ‘Submit’ and then go to the command prompt. Now I’ll ping this IP address: 127.0.0.1.

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, 127.0.0.1, is the lowest loopback address.

This one is the highest: 127.255.255.254. And there are the responses.