Security :: Restrict Access To Site From Outside Office?

I am faced with a rather tricky issue. I am developing a web application that resides beneath a web site. The web application is actually meant for the employees of the company owning the web site. The employees can access the web app from the login facility on the site.

The situation demands that an employee must be able to login to the app only from the office machines and not from anywhere outside. I thought of a logic where in the IP address of the machine in which the employee sits will be stored against the employee profile and when he logs in, the authentication will check for user credentials as well as whether he is logging from the designated IP. If not he is not allowed access to the app even if the login credentials where correct.

I am not sure if this is a good way, because I feel tricky persons can give the same IP of the office machine in another machine, say at home and the logic is broken. Can somebody provide me a better way of solving the issue. I am using ASP.Net login control for user login.