Firewalls
Imagine that you take your cell phone to a room where, once you are inside, you cannot receive incoming phone calls from outside the room, but you can make outgoing calls. Once you dial someone outside the room and they answer you can carry on a conversation just like any other phone call, but since they are outside the room you have to call them, they can't call you. Now let's imagine that if more than one person is in this room with their cell phones they can call each other, and they can call to anyone outside the room also. In other words, nobody outside the room can call a phone inside the room, but the people inside the room can call anybody whether they are inside or outside the room The room doesn't block phone calls, it only blocks incoming phone calls.
This is exactly how a firewall works. Let's run through an example. Say you have two computers attached to a router. The router is also connected to the internet via either a DSL or cable connection. Your router probably has a built in firewall that works (if you've connected everything together correctly) like the magic room described above. The two computers are "inside" the firewall (like two cell phones in the magic room) and the internet is "outside" the firewall (like all the cell phones outside of the magic room). Now you open a web browser on one of your computers. It initiates a session (makes a phone call from inside the room) with a web server on the internet (a phone outside the room) and data flows in both directions.
Let's say, further, that you have a shared folder containing music files on one of the computers, making it a file server The other computer can bring up a client and play those music files because both computers are inside the firewall, just like people inside our magic room can call each other.
But if a computer from the internet, outside the firewall, tries to access those music files, they can't. The file server is still sharing the files, but computers on the internet outside the firewall are blocked from initiating a session with a computer inside the firewall, just like phones outside the magic room were blocked from making phone calls into the room.
There's a lot more to firewalls, but the above gives you a basic understanding of how things work. One last point about firewalls: there are two places where you'll probably encounter them. One is in the router as we've been discussing, and the other is a software firewall inside your computer. All of the recent versions of Windows come with a firewall, and if you have any antivirus software running it might have replaced the Windows firewall with its own. These software firewalls work exactly like the firewall within your router.
If you're concerned about the security of your network,
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