Get IP Address of request in an XML Web Service

Righty. I have an XML Web Service that checks stock and prices directly through to our suppliers. I'd like to ensure that only requests from localhost are processed - i.e. do not allow anything to query the DB except the website itself.

I cannot however access the Request object from inside the web service, so how can I access the source IP?

I've actually moved a load of common methods etc. into one single web service that's residing inside a different project. So I can now specify through IIS the access rights to that folder :)