I don't know much about cable internet, but FIOS (Verizon fiber optic to the home for TV, Telephone, and Internet) assigns IP addresses to FIOS devices that aren't normally part of a home network (digital video recorders, tivo is an example, and set top boxes, similar to cable boxes).
What I'm getting at is that you might have other devices on your network with IP addresses. Devices, that aren't considered networkable by you, but are still part of your network. I was kind of surprised to see the IP config info in the configuration panel of my DVR and set top box. I was even more surprised to see how effing chatty they were.
Chris