Ever want to know who they are? If you have a good firewall, then don't worry but it's still fun to know (sometimes).
You will need the IP from your firewall. It should be something like IP xx.xxx.xx has been blocked or something similar.
If you have WinXP
Go to Start, Run and type "Command" (without the quotes) hit enter
You should now see a DOS screen
type nslookup (and the IP) hit enter
It should say the IP xx.xxx.xx is for ____________ , (sometimes it even gives you your IP info first).
Now that was kinda cool but wait, theres more, you can even do it backwards.
Type in nslookup (and the name of what you want to look up) and hit enter
it should give you the IP address (again sometimes it will give your information first)
Just a side note; I had to teach this to some web owners because they forget their address. LOL Have fun!