Why ???

1) You enter a website. Once you hit any one of the files on the webserver, the website owners can find out these pieces of information about you, and much more:

1. Your IP Address.
2. Your hostname.
3. Your continent.
4. Your country.
5. Your city.
6. Your web browser.
7. Your Operating System.
8. Your screen resolution.
9. Your screen colors.
10. The previous URL you've been to.
11. Your ISP.

And this is just the tip of the iceberg. Go to our homepage at blacksun.box.sk and find the web statistics button (later addition: we have terminated our account on our webstats provider because they were quite buggy, and we've decided to use a php3-based text counter). There you will be able to see how much we can tell about our visitors

2) Another example: you're connected to an IRC network and you are chatting with your friends. Right now all a person needs to find information on you is nothing but your nickname. He doesn't even have to know you, or be in the same channel/channels you are. Here are a few examples of what you can find by simply knowing a person's nickname (in the most optimal conditions):
1. Your real name.
2. Your Email address.
3. Your IP address.
4. Your hostname.
5. Your ISP.
6. Your continent.
7. Your country.
8. Your city.

And much much more.
The same goes for online games that allow players to view the other players' IP addresses.

3) Suppose my name is Paul Matthews, and my Email address is pmatthews@boring.ISP.net. It is extremely easy to figure out that the first letter of my first name is P and that my last name is Matthews, but that's not all. Some ISPs give their entire listings to web directories. Meaning, people can go to, say... whowhere.com, punch in the words Paul Matthews or search for people with Matthews as their last name on boring.ISP.net and find out that pmatthews@boring.ISP.net does actually belong to Paul Matthews, hence discovering your real name. But it is also possible to use these web directories for 1,001 uses. Therefore you should go to whowhere.com as soon as possible, try to track down yourself and then tell whowhere.com to delete your listing.

4) Some ISPs also run finger daemons. A daemon is a program that waits for incoming connections on a specific or several ports. The finger daemon is a daemon that waits for open connections on port 79. Once you get in, you need to punch in a username on the system the daemon runs on and you will get tons of information about him. For example: a while ago my ISP was running a finger daemon on their servers (until I forced them to take it off because it was a privacy invasion). Now, suppose you know nothing about me besides my Email address, which is barakirs@netvision.net.il. The first thing you should do is to go to netvision.net.il on port 79 and hope there's somebody there. If there is, you can find the following information by typing in my username, barakirs:
1. My real name.
2. When was the last time I was online.
3. If I'm online right now, since when have I been online.
4. Whether I have new mail or not.

And much much more (some finger daemons might give out any pieces of information, such as my home address and phone number). Besides the obvious uses (finding a person's real name and other private information), you can use this information for various purposes, such as:
1. Most instant messangers, such as ICQ, AIM, YAHOO Instant Messanger and MSN Instant Messanger, allow you to add people in or outside your contact list to an "invisible list", so they won't be able to know whether you're online or not and you'll appear to be offline to them. If they have your Email address, and your ISP is running a finger daemon, they are able to know whether you're really offline or just trying to fool them.
2. Your friend promised you to do something for you on the net, but when you finally go online to ask him if he's done it he says that he just got back from work and that he just got online. Using finger, you can test this and see when he really got online.

These were just a little out of many examples.
During this tutorial I will explain to you how to prevent people from finding out information about you (there will always be new tricks, but blocking the most basic / common ones will hold off most attackers and make it harder for the more experienced ones). If you really wanna learn how to do these things, as well as some really cool and advanced tricks, then read the 'Info-Gathering' tutorial.