5.1 Dynamic Host Configuration Protocol (DHCP)

So let’s see how it works. Here’s my ‘Home’ network over here on the left. I’ve also got ‘Work’ network on screen as well, but I’ll talk about that in a minute. Let’s focus our attention on the ‘Home’ network here on the left, where I have a typical small domestic router. It happens to be a Wi-Fi access point as well, but I’m going to start just using wired connections, but actually everything will work just the same because DHCP will work across different transport layers.

So let’s see what happens if I bring my laptop into the network; let’s say I’ve just bought myself a new laptop, which will need configuring. So I’m going to click on it to configure it. But instead of typing in an IP address as we’ve done previously, I’m just going to click ‘DHCP’ and allow it to get configuration automatically. Nothing is going to happen until I actually connect it though, so let’s get a cable and connect from the laptop Ethernet port to the router – pick a free Ethernet port – and then I need to sit back and wait for the configuration to take place. So DHCP is a protocol that means a conversation, an exchange of messages between the devices – the laptop and the router – until they’ve between them sorted out what the correct configuration is. So I need to be patient while those messages are exchanged. And this is Packet Tracer and that is simulating all that exchange of messages, so I need to wait for that to happen. The lights have now gone green, so hopefully I can just check that the configuration is what I expect and I now have some values in here. And if I open a command prompt I can type in ipconfig and check to see what my configuration is. So this laptop has been given a configuration. It’s been given an IPv6 address, which we’re not going to talk any more about. It has an IPv4 address, which is 192.168.0.100 and it’s been given a subnet mask as well, 255.255.255.0, and a default gateway, 192.168.0.1. Now, the default gateway is the gateway to the internet – in this case it is the router that we’ve just connected to. So I can just check to see that that is working by doing a ping to the router, 192.168.0.1, and just check that we do get an exchange. And there we have some pings going backwards and received so that has a good connection. So that’s looking very good.

Let’s go wild and have a second laptop in the network. I’m going to do the same configuration again, just turning DHCP on and that’s all. I’m going to get another cable and I’m going to connect from that laptop Ethernet port to another free Ethernet port on the router. And again, I’ll need to be patient while that configuration takes place. Hopefully, I’m going to get an IP address; hopefully, I’m going to get a different IP address from the first laptop. So two different devices are going to need two different IP addresses to work on the same network, otherwise confusion will reign. Lights are still amber … there we are, gone green now. So I’m now going to open the command prompt on this laptop, do ipconfig on the second laptop, and we can see that we have an IP address. The IP address is different: it’s 192.168.0.101, so it’s different from the first one. The same subnet mask, because it is on the same subnet; the same default gateway because it is going to use the same router as a gateway to the internet, if the internet was connected. And I can check now that I can ping from the second laptop, which is where I’m sitting at the moment, to the first laptop: I can do ping 192.168.0.100 was the address of the first one. And there we are, we’ve got a reply from 0.100, some successful pings so I have got a path between the two laptops through the router acting as a switch.

So that’s all looking very good. And I could go on adding new devices, mobile devices using Wi-Fi – it would work in exactly the same way.

What would be interesting now is to see what happens if I disconnect my laptop from my home and take it to work – bring it up here and take it to work. Look for another connection, so let’s get another cable and go from the Ethernet port to a spare port on the router. And then again I’m going to need to sit back and be quite patient while the DHCP on that router kicks in and gives me a configuration. While it’s doing that, I think I might just point out you might think there’s a bit of a problem here because the IP address is at the moment either not set or set incorrectly because it was on the wrong network. So how is communication happening at all? Well, the answer to that is that it’s working at the Ethernet level because it’s able to do that just within the local network because each device has got a unique MAC address, so Ethernet communication can work without IP addresses being correctly set. And it can be done as a broadcast: the laptop can just broadcast and say ‘is there a DHCP server out there?’ and then the DHCP server will reply and start the conversation that leads to configuration.

So let’s check to see that I have got configuration on the laptop now. I’m going to do ipconfig again, which was set at 192.168.0.100 at home. Let’s see what it is now. Ah, it’s changed. So we now have an IP address which is 172.16.0.100, so that fits with a different network; has a different subnet mask, and a different default gateway because I’m now talking to a different router, 172.16.0.1. And so that’s all looking good. So you can see that DHCP is able to take a dynamic host – take a host from one place to another – and sort out the configuration all automatically: there was nothing that I needed to do to type that in.

So there we are, DHCP (Dynamic Host Configuration Protocol).