Three days ago an old friend/enemy stopped by, they need to use the internet, then asked for my password, which I gave him. After he left, I contacted my provider and changed my wireless system name and password. My questions are, can I find out the IP address of the computer he used? does my modem/router have a history or log, from before I changed my password?
Thanks for your question Chrlyangelpi.
The External IP he used would be the same as everything else on your network. That's the IP you get from your ISP.
His Internal IP would usually be the lowest number available in the DHCP pool unless he configured a Static IP which is not likely or unless the lowest number available is being held by an open lease for some other device that wasn't on at the time.
Most residential Routers have the ability to log traffic but unfortunately, most don't have internal storage so logging is usually off by default. It can be enabled but it needs a computer to send the info to so that computer must be on all the time. If you are using a computer as a Router with some Nix based Operating System, it probably does have a log and it would usually be enabled by default.