Well, then this already is the answer. Comcast does not allow incoming port 80 traffic for homeservers. You basically have three options:
a) Put that webserver on a different port, e.g. 8080
b) Switch ISP
c) Get a real server
Option a) is probably the right solution, if you just want to serve semi private stuff to friends. Option c) is what you should do, if you are looking into having a real website. You can't seriously run these on homeservers anyway.