User Activity

"IPCONFIG /ALL (in a command line in Windows) will tell you under the heading Default Gateway." Then I logged in, and changed my modem's password. Does this change my external IP address? 
Your modem/router might keep logs on which IP the DSL was registered to. Usually the modem's logs/UI is accessible by navigating to its IP through your browser. How do I access these “logs” of all past IP addresses? Which tool below shall I click?