To start out, I’d like to describe the images and icons that you see here at the top, starting with the one on the left. That is the logical representation of a router that we use all over the place in topology, maps and network diagrams. It is a very widely used icon. The little sort of cylindrical or circular shape there with the four arrows that describes a router. Now that’s logical. So what is a physical router? Well, the image you see below is what a physical router can look like. This particular router is a Cisco 2900 series router that we would use in enterprise grade and business grade networks. Now, keep in mind, not all physical routers look alike. They come in different varieties however one thing they do have in common is their purpose.

What a router does

What is the purpose of a router? The purpose of a router is to route traffic from one network into another network. If I’m sitting here in my own LAN and I want to communicate with and send traffic out to the internet, I need to send that through a router or some type of routing service or routing device. So that’s the purpose – to take traffic from one network and allow it to route to another network.

Most common router

The most common type of router is an IP router. There are other protocols out there besides IP that we can actually route from one network to another. Internet Protocol networks are the most common type of layer three network out there. An IP router is the most widely used type of router. Any router you find out there, including the router at your house, runs IP.

IP routers route IP packets from one network to another network. If I’m sitting here on my network and I need to communicate out to the internet using IP, I’ll send IP packets out to let’s say, a Google server for example. My home router will then know how to route my packet out onto the internet. That’s its job.

Let’s define this a little bit further with our diagram. What you can see here we have two LANs. These are two physically separated LANs. We have two different switches connecting LAN one and then another switch connecting LAN two. The host on LAN one can communicate with hosts on line one. The hosts on LAN two can communicate with the hosts on line two, but they cannot communicate to each other. They’re physically separated in their own different networks. In order to get traffic from LAN one over to LAN two, we need some type of router service. We would need to connect these switches into our router each on different router interfaces. As long as the router interfaces were configured with an IP address on those networks, then it would automatically be able to route traffic from one LAN to the other LAN and allow LAN one to communicate with LAN two and vice versa. 

Here we can see more of a physical representation on the diagram of what I’m actually talking about. Because routers are able to route traffic from one network to another network, they are said to be the gateway device or the network gateway. So, if I’m sitting on LAN one, and I need to send traffic out to another network, I need to send that to my gateway device, which is going to be a router. In fact, most of the time we call this the default gateway in networking. If you’ve ever heard that term before, the default gateway for a network is normally talking about a router or some type of device that can do routing. It could even be a firewall; firewalls can do routing most of the time. The default gateway is an important term. We hear it all the time. When you hear that term default gateway, it’s talking about the router that connects to your LAN.

Networks

Earlier we said that our router connects us from one LAN to another LAN. But are there other types of networks out there besides local area networks or LANs? Yes, of course there are. Now we have LAN one and LAN two here. But what are some other types of networks or what is another type of network that I might really want to be able to get to? It’s the internet. Our routers will connect to the internet as well. 

Connecting to the internet is going over a different type of connection. It’s not a LAN connection or a local area network. It’s a Wide Area Network Connection. Even at your house your router connects to your internet service provider that goes over a WAN connection in order to connect you to the internet. We can also say that routers could connect the LANs to WANs. In fact, they can connect to one LAN and to another LAN over a wide area network. These are other common terms of networking. This is what routers allow us to d – send traffic from one LAN to another LAN and communicate from LANs or WANs and between all our different types of networks.

Routers are very versatile, and they’re a necessary tool or device in our network. The internet would not work without routers. In fact, the internet is a conglomeration of many different networks and routers connecting all these networks together, a super important thing. As a network engineer, this is one of the main devices that you need to know how to configure, set up, connect, troubleshoot, and install. Alright, so because our routers are devices that connect to our external services, we also use them to connect to other things besides just the internet.

Interfaces

Looking at the backside of a router, we can see on the right side of it that we have a few different Ethernet interfaces. We have three on this model router. We have three Gigabit Ethernet interfaces. On the left side or all across the back, we have these interchangeable slots for line cards slots. We can literally remove the plate on these and insert a different type of card in there to give us another type of network connection. For example, we might not be using Ethernet to connect to our WAN service, we might be using ATM, or we might be connecting to a T1 connection. Or maybe we have a circuit from a voice service that we need to connect out to the public switch telephone network to provide voice over IP services to our network. In order to do that we need the type of connection to interface with that. We can pull out our slot here or we can take a line card that’s made for that and put it into a slot on the router. Routers are super versatile and they allow us to connect to all of our different wide area network connections and connect to our different service providers to provide us connectivity across the long haul.

Home routers

Our solo routers are just hardcore routers and that’s all they do. If you are at your house you might be thinking, well, do I have a router at my house? I have a device that connects to the internet. I have multiple computers that connect to it. Is that a switch? Is that a router? I think I have wireless too. Is it a wireless access point? It’s actually all of the above. Your home router or your Soho router, small office home office router is actually a combo device. It’s a router, it’s a switch, and it’s a wireless access point. Most of the time people might have just a solo router connecting to a solo switch and a solo wireless access point, if they do, they’re probably networking guys. You can have that still at your house, of course. But normally, whenever you go out to Best Buy or something like that and you buy a router or device for your house to connect to the internet, it’s going to be a combo device. But yes, it is also a router. It also does have security features or firewall services. Most of our routers out there do allow us to do some type of firewalling because they allow us to connect to the internet and if we connect to the internet, we want to protect that connection. 

Conclusion

All right, that’s a router in a nutshell for you guys. You should now have a good understanding of what the purpose of a router is, what a router looks like at your house, and how it actually is doing multiple functions being a router and a switch, and also a wireless access point. Just to remind you guys in the enterprise world and in the business world, normally you’re going to have a standalone router that just does routing really, really well.