>I have a wireless router in my office. My office is in an office building (of small companies). Sometimes my internet connection is very slow. I was thinking, what if someone using my router? Is there a way to see/find out it?

There are a couple of things you can look at:

- many routers have a "LAN activity" LED that flashes when there is traffic to/from the local network. If your computer is switched off, but there is still flashing that *may* indicate other people using the router/connection

- If someone else attaches via DHCP, this should be visible in the router's configuration pages. Most offer a page that shows current "DHCP clients". If there are more IPs in this list than legitimate/switched on computers in your office, someone else may be attached. Of course, if someone else sets up a fixed IP address they won't appear on the DHCP clients list.

- You can manually try to "ping" other IPs on your local network. For example, your router may be 192.168.1.1, your computer may be 192.168.1.100. From a CMD prompt you could try

PING 192.168.1.xxx
* where xxx is everything from .2 to .254 except .100 (which is your computer)

If you get a response other than "Request timed out" then that is another active computer on your network.

None of these methods is 100% reliable. Far better is to secure your network so no-one else can use it. Implement WPA-PSK encryption (or WPA2, if your router and wireless NIC support it) with a strong password (not a dictionary word, at least 12 characters, mixed upper, lower case and digits). Do NOT use WEP - it has been broken, anyone can hack into a WEP "protected" network in a couple of minutes.