how to restrict the users from the outside domain for website which uses forms authentication ?

We have a website which is exposed to internet with asp.net MVC 3 with forms authentication.
The users are not accessing the website using their domain emailds , they have seperate usernames.
Here our requirment is to restrict the users who are accessing from oustside the compnays domain/Network
So we need to check the request is coming from the companys domain even if the users are accessing outside the company or inside network, since users has the citrix or VPN access to the companys network/domain -Please correct me if I am wrong
Is it possible to get the requesting users domain if they are accessing from outside?
if possible will it give the domain while the users are accessing using VPN/Citrix ?
How to check the users domain . whether it can be checked by writing custom httpmodules or httphandlers?
Is it possible to restrict by doing configuration in IIS only for this website?
Please ...