There are a few ways. You can go into the TCP/IP settings
of your network connection on the internal. connected
adapter. There you can go in to the General tab, then
advanced, then options, then click on tcp/ip filtering.
After click on properties then go to enable TCP/IP
filtering and do the permit only port 80. This can be a
bit too restritive.
Another way is to use a Cache server or proxy to filter
the destination port to only port 80. The more common
Caching servers are better. Do a google search.
