From the previous sections, it should now be clear that routers play a key role in regulating and making possible the data flow on the Internet based on the TCP/IP standards. Indeed some routers are big, costly, complex pieces of hardware located in key nodes of the world network (Figure 1-3-1).
However, inexpensive, light weight routers are now available, from many different manufacturers, that allow the easy creation of an home or office Local Area Network, and the connection of this LAN to the Internet (Figure 1-3-2). This second step is optional, it is perfectly possible, with such routers, to create a private LAN not connected to the internet. Some models will allow the creation of more than one LAN (for example a private LAN reserved to lab members and a guest LAN, with a printer and internet access), but we will not discuss this here.
Home routers have several ethernet ports on the back. Typically 5 for smaller models (Figure 1-3-2). One of these ports have the purpose of connecting the router to the Internet. The other 4 allow the connection of network devices such as computers, printers, scanners, network drives (for storage and backups) and more. All these devices of course support TCP/IP. In addition to connecting the devices via ethernet cables, more devices (or all the devices) can be connected through a wireless connection. From the network point of view there is no particular difference between a device connected by wireless and a device connected by cable. They are both connected to the router and belong to the same LAN.
In a typical scenario, the router is configured as DHCP server, which means it will provide a LAN IP address to any device that is connected to it, either by a wire or wirelessly (Figures 1-3-3 and 1-3-4). This address is a local address, only visible INSIDE the LAN. This address, that might be something like 192.168.1.24, is not a public internet address, this is a crucial concept to understand in this discussion.
The router configuration can be fine-tuned in the router administration interface. This is accessible from any computer connected to the router, for example through a ethernet cable connecting the computer to one of the LAN ethernet ports on the router. No internet connection is required at this time, just a cable between the computer and the router. The router admin interface is accessed through a browser, by typing the IP address of the router (Figure 1-3-4).
Please note that the router, at the end of the day, will have 2 different IP addresses (see Figure 1-3-3), one “for the inside” of the LAN – the router will be the gateway of the LAN so this is known as the gateway address – , another one for the Internet. To connect to the admin interface of the router we need to type the internal/gateway address. This will typically be something like 192.168.1.1, that is, on the LAN, the router will reserve the number 1 (the last number in the IP address) for himself. It can then assign one of the other 244 addresses available (from 192.168.1.2 to 192.168.1.225) to any device that is connected to the router’s LAN.
Devices inside the LAN can see each other through their internal LAN addresses, the ones assigned to each by the router when they were connected. You can think of a LAN as a “small Internet”, where the same rules and protocols (TCP/IP of course) apply, as in the “real” Internet. Inside a big LAN (say a department or Faculty LAN) you could have web servers, mail servers, FTP servers and all the goodies you might expect on the Internet, even if the LAN is totally disconnected from the Internet. All the services (for example websites) would be available only to those connected to the LAN. Such model is sometimes referred to as an “Intranet“, and it is commonly found within organizations of every kind, including academic and research centers.
With an home router, you can easily create your own home or lab LAN, which allow connected users to share a number of internal resources such as for example printing, and to connect to the internet. If you do not really need the LAN for shared internal resources, you can still use an home router as a connectivity multiplier. If you can connect it to one ethernet port (maybe the only port you have available), you can then use it to connect up to 244 devices (for most basic models) to the internet, through this single and only ethernet port.
With this basic knowledge and maybe some digging into the router available options and the router’s manual, you should be able to setup a lab lan for your computing needs and those of your co-workers.
You might wonder, if the internal LAN address of a PC is not available/accessible from the Internet, how can you possibly send requests to Internet servers and get a reply at your (invisible) Ip address? How can you browse a web page? The answer lies in a mechanism known as Network Address Translation (NAT). The router uses NAT to send out your requests with his own public address. When the return data arrives, the router applies NAT to route back the data to your IP. This is a technical aspect beyond the scope of this tutorial.