Think your home or small office wireless network is safe? Do you have any idea whether there are any intruders on it? This free program is a great way to find out. It's a bit technical for novices, but will do no harm if you decide to use it as a learning tool.
Run RogueScanner on a PC attached to your network via Ethernet. (The program won't work in Windows Vista, so you'll need Windows XP.) When you run it, it immediately does a network scan, finding the wireless and wired devices attached to it. For each device, it lists the unique MAC address (a kind of serial number), the IP address, the manufacturer if it can figure it out and model number if it can find that. It also tells you what kind of device it is, such as a router, printer, PC, and so on. And it will also flag any suspect devices it finds.
Just compare the information it finds to what you know of your network. If there are any wireless devices connected that you don't know about, you may have an intruder. Get RogueScanner here: http://www.networkchemistry.com/products/roguescanner.php
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This is also a quick way to gather all your IP addresses in your network.