Block and unblock a website

Sometimes,eventhough we have access to the internet in colleges some sites like orkut etc are banned.

Now, the 2 qts that arise in mind are
1)How is that blocking done??
2)How to overcome and unblock??

Blocking a website:
For eg you want to block www.xyz.com !

1) Open the folder C:\WINDOWS\system32\drivers\etc

2) There you will find a file named HOSTS

3) Now,open that file using notepad(right click and open with notepad)

4) Now, in the file under the line 127.0.0.1 localhost add another line as 127.0.0.2 www.xyz.com.

5) Now, File>>Save !

Now if you try opening www.xyz.com,it will not load

Unblock a website:
To unblock a site,just remove the line added in HOSTS file.(sometimes you may need to restart to load the site)