I run into this situation occasionally with business clients (typically in
small office buildings) who provide Internet connectivity to tenants but do
not want them to have other network access. The best solution to is to
segment the network using one or more routers. You would put a router
between the Internet only user and your network. Set the gateway address on
the router to point to the Internet connection (which will be your server or
router).
In your case it's probably the only practical solution. If you were
running a domain server or didn't have the XP HOME machines in the loop,
there could be other options using access control lists. XP HOME, because
it is limited to simple file sharing, rules that out.
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