6.5 Routing

The one job the gateway router hasn’t done yet is act as a router!

We need to connect it to the rest of the internet. At home, that typically means either a broadband connection through copper phone line or an optical fibre connection, maybe in some rural areas 4G radio or even satellite.

Whatever, it won’t be an Ethernet connection, so a device, a modem, is required to convert signals from one system to another. Home gateways often include a broadband ADSL modem that connects to the phone system – it’s another role for the gateway. In this simulation, I’m skipping that detail.

But where are we connecting when we do connect to the internet? It isn’t really a cloud – we will really connect to another router, one that belongs to our ISP. That will take our traffic and be responsible for forwarding it out to the rest of the internet, from router to router until it is delivered to the destination host. Reply packets will hop back over the internet, eventually coming back to the ISP router, which then delivers back to our router which will deliver back to our device.

So I have made a connection from the gateway router to the ISP’s router. Now, at this point our own gateway router itself will be configured by DHCP; it will receive an external IP address from the ISP’s DHCP server. (So this is it working in another role – the gateway router itself is working as a DHCP client, getting its external IP from the ISP’s DHCP server, while also acting as a local DHCP server itself for the devices in the LAN.)

Note that the gateway is a router. A router always has two network interfaces each with its own IP address. One faces the LAN and is used for traffic on LAN only. The other faces the internet and is used for traffic between the LAN and the internet. The router’s job is to decide which packets to forward from one interface to the other.

I’m going to go back to desktop and check its configuration. We need to make sure that it knows where to find the gateway router. I can check with ipconfig and see that the gateway is correctly set.

And I can now ping outside my home network to a server on the internet. I happen to know that the IP address of the Megacorp web server is 77.0.0.2 – and there after some delay we finally have some return packets. So it’s taken a little while for the routing to sort itself out.

I can open a web browser on my desktop to show that we can do something more useful than simply ping – I can reach the Megacorp server by giving its IP address. And there we can see its web pages.

The reason that we are calling this gadget a gateway is because it does act as the gateway from the LAN to the rest of the internet – all internet traffic is routed through it. And finally the gateway router is doing the job of a router – it’s forwarding traffic from one network to another, from the LAN to the internet.

In passing we should note that my desktop has a private IP address, like everything else on the LAN, but it is successfully communicating with the internet. That means there must be NAT (network address translation) going on in the gateway – so that’s another role that the gateway is carrying out.