To make sure that the only devices installed on your network are the ones you put there, take advantage of the free scanning tools that are available
Are you absolutely sure you know all the protocols and ports that are open on your network? If you're not the only person with the rights and permissions necessary to add devices to your network, you'll never know what's really "live and on the wire" -- unless you listen to your network. By periodically scanning your network, you'll be able to maintain a good view of what devices are connected to it and to determine whether those devices are communicating properly and using the allowed ports and protocols.
http://insight.zdnet.co.uk/internet/security/0,39020457,39145260,00.htm

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