It’s easiest to think of a computer on a network the same way that you think of a house in a neighbourhood.
- A house has a unique street address—a computer has a unique network address.
- A house has entry points of doors and windows and pipes—a computer has entry points called service ports.
- A house may have natural gas continuously delivered via a pipe, or periodically delivered filling a tank—a computer has information delivered to it using different protocols.
What’s an Address?
For our purposes, a computer address is comprised of four numbers, ranging from 0-255. So the range of addresses is 0.0.0.0 to 255.255.255.255. A good example of a computer address that you have probably seen is 192.168.0.1.
The analogy actually goes further—before a house was even built, the tract of land was probably identified by a lot and concession number (depending on your planning department). This is a unique geographic location for the land.
A computer similarly has a unique network adapter identifier called a MAC (Media Access Control) address. It is comprised of 6 hexadecimal numbers, and looks something like 00:50:55:7F:D3:56.
Just like the planning committee assigned a street address to the lot and concession number, a computer network assigns a unique network address to a MAC address of a given computer.
This numbering system (IPv4) is changing and expanding because we have run out of addresses to accommodate all of the computers and devices in the world! (See IPv6 for more info.) Today, however, we’ll stick to this numbering scheme.
So a computer address really is very much like a house’s physical address.
What’s a Port?
There are typically doors and windows, chimneys and pipes, and other entrances and exits in a house. A computer also needs to move various types of information into and out of the computer. It does this by using ports.
In a house, you expect to get water from a faucet and electricity from a wall socket. Similarly, the computer assigns specific ports to get different types of information, like email, files shares, and for browsing websites. Just like people share services within a house like water and electricity, programs in a computer can share ports to get and send information like email and web browsing.
Ports in a computer are simply numbers, from 0 to 65535. The ports in the range 0-1023 are called “well-known” ports because these numbers are assigned to provide specific well-known services. Just like you expect electricity from a wall socket or water from a faucet, a computer expects a website to be at port 80 or 443, and files to be moved via FTP at port 21.
Certainly, other services can be provided through a variety of pipes and wires in a house, and other information services can be provided at other ports in a computer.
What’s a Protocol?
You have probably heard of TCP/IP, but have you ever heard of UDP? These are two examples of information delivery protocols that computer networks use to move information.
It’s very much like delivering natural gas to a house—a pipe delivers a continuous stream of natural gas, just as TCP delivers a stream of data. UDP is a lot like filling a tank of natural gas. You get it when you get it, the order in which you get it might get mixed up, and you may even miss a delivery.
There are many more network protocols, but TCP and UDP are the two that we will worry about in the firewall configuration.
That’s it! That’s all the technology you need to understand in order to configure a firewall. By making a house analogy, it doesn't seem so hard. In fact, as we get into the details of configuring the firewall, we will see that a lot of the precautions we can take to protect a house have analogies with securing a firewall and computer.