I had this problem in many houses I get called out to..
Best way to solve this is disable all network cards on every computer..
One by one, enable the computers networks. Connecting them to the internet.. Make sure you do this one by one..
If you find one computer trying to grab a peticular IP then start over and enable that computer that is giving you the trouble 1st.
"The Crazy American says.."
A well regulated Militia, being necessary to the security of a free State,
the right of the people to keep and bear Arms, shall not be infringed.