Electronic devices in a local area network (LAN), e.g., a DVD video recorder, plasma display, and audio controller in a home network, are protected by a security system that is networked with the devices. In one aspect, the security system periodically polls the devices to confirm that they are present in the LAN. If a device does not respond, an alarm is set. Installation of the electronic device in an unauthorized network is detected by verifying the Internet Protocol (IP) address of the device, such as when the device attempts to contact a server that provides services such as downloading new or updated software to the device, performing remote programming, and uploading diagnostic data. The devices may encrypt their messages using encryption codes that are unique individually or for a specified group of electronic devices.