Accessing a Web Page hosted externally

I have a client who is running Win 2K in a domain environment. Their webpage is hosted externally by a 3rd party. Whenever they try to access the page while in the office they are unable to, however there are no problems accessing it from outside the office (although I am unable to ping it to resolve an address most likely due to the host’s security). My thinking is when they try to access the page, since they are w/in the domain environment, it is looking to the server and not beyond it. Is this assumption correct and if so, ho do I “forward” it to the webhost?