Yes, and if the interloper does something wicked, you will go to jail or be harassed by your [spit] RIAA ...
Use strong encryption (WPA2 if you can); use the longest password you can randomly generate and use; view your logs frequently -- if you see repeated "association" requests from a MAC address that you don't own, ban it if if you can (not totally effective, but not a bad idea); that kind of stuff.
Really, so when they connect to the net from my wireless they are actually connection using my IP address.? if so that mean they can do all sorts of dodgy stuff and no one knows who they are.! is there anyway of finding out who has been using it and where?
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You have two IP addresses. One is assigned by your ISP to your unspecified model wireless router (WAN IP). The other is assigned by this router (LAN IP) to your various desktops, laptops, and whatever. There are about 253 of these addresses available, none of which are unique to your system. A hacker can assign their own without much difficulty. The way NAT (network address translation) works is that everyone, including the hacker, is sharing the same WAN IP address assigned by your ISP.
Can't help without a clue as to what you have in the way of equipment. Many wireless routers have status pages that display all wireless connections. Unfortunately, many do not. For those, you can sniff the traffic between the router and the DSL/cable/satellite modem with something like AirSnare:
There are plenty of articles on wireless security. In my never humble opinion, most of them go off the deep end with creative and often useless security measures that mostly consitute an obstacle course for wireless intruders. Your only real form of wireless security is WPA or WPA2 encryption. Use a long (>20char) radomly generated shared key that's not in any known dictionary. It will provide all the security you need to keep most evil hackers (like me) out of your system.
If this is insufficient, then there are additional measures of securing this shared key to prevent someone from extracting it directly from your computers. You can also use the aformentioned intrustion detection program to verify that the encryption is doing its job.
Incidentally, next time you ask a question, try to format it as:
What problem are you trying to solve?
What do you have to work with? (hardware, software, versions, etc).