determining where the computer is that posts to a db

I have a time billing application and I need to determine where people are logging in from to make sure they are signing in at work and not at home. Currently I'm logging the IP address and hostname but I don't think that's the totally sano way to do it. The work sites are using local ISP's that register dynamic IP addresses. If one of the workers has the same ISP I can't tell whether they are at home or work. Any ideas? Is there a var that could get the MAC address of the machine? Thanks, Tom