I think you meant the internal and not the external.
It changes because of the DHCP server (Dynamic Host Configuration and Protocol). The DHCP works with ranges: 192.168.1.2 to 192.168.1.254 (example). If you want a static IP address, just change it to 192.168.1.3 to 192.168.1.254 (or whatever it is), and make your IP address: 192.168.1.2.