I frequently use something called IP Geolocation to find out where a user is located for forms, enquiries, or any log evidence of someone doing something to try to establish if they are who they say they are.
It does not work 100% but I would say it works in 99% of cases. The router or gateway you use that your network/ computer is plugged into has an address allocated to it by your service provider (BT, Talk Talk, NTL, etc). For most of us the IP address changes from time to time and is referred to as dynamic. They do this to make sure it is hard to run your own web server from home. If and when you reset your router, when it comes back online it will likely have a new IP address. You can find out what your IP address is by clicking on this link. Continue reading IP Geolocation and why it may be important