maybe this will help? I know RCN used to block port 80 inbound, but that was years ago, and supposedly a temporary measure, in corporate speak, during one of the virus attacks. But who knows.
Detecting Ports Blocked by Your ISP
Internet service providers often block specific traffic entering their network before it reaches their customers, or after leaving their customers before it exits their network. This is sometimes done to block the exploitation of common security vulnerabilities, and sometimes to prevent their customers from offering proscribed Internet services.
As a customer, it can be useful and interesting to know which service ports, if any, an ISP has chosen to preemptively block in order to restrict their customers' global Internet traffic.
ISP port blocking can be easily tested, often quite rapidly, by arranging to allow the ShieldsUP! probe to have access to an unprotected computer. Since all non-stealth machines will respond to every open request - either affirmatively or negatively - ports appearing as STEALTH will be those blocked by your ISP, corporate firewall, or other external agency.
If your system is unprotected, without any personal firewall or NAT router, any ports showing as stealth are being blocked somewhere between your computer and the public Internet. This is probably being done by your ISP. Internet traffic directed to your computer at the stealth ports will be dropped before reaching your machine.
If your system has a personal firewall that can be instructed to "trust" a specific remote IP, you can temporarily instruct it to trust the ShieldsUP! probe IP of [184.108.40.206]. If, after doing so, most of the service ports change to either open or closed , you have succeeded and any which remain stealth are being blocked by your ISP.
If your system is operating behind a residential "NAT" router, the router will be acting as a natural and excellent hardware firewall. But that's not what you want for the moment. You can temporarily remove your NAT router and connect an unprotected computer directly to your cable modem or DSL line. Or, if you are comfortable reconfiguring your NAT router, you may be able to point the router's "DMZ" at one of your computers which has been instructed to "trust" our probe IP of [220.127.116.11]. If, after doing so, most of the service ports change to either open or closed , you have succeeded and any remaining stealth are being blocked by your ISP.
Finally, if your Internet security system, NAT router, personal firewall, or whatever, can produce detailed logs of incoming Internet packets, you could leave your existing security in place, clear your log, run the service ports scan, then carefully inspect your log for any consistently missing port probes. We send out four sets of probing packets because individual packets are sometimes dropped along the way. Therefore, it won't be unusual to see occasional missing packets from your logs. What you're looking for is a complete lack of packets bound for a specific port. A careful and detailed examination of your log will reveal any missing ports which are being blocked before they reach your logging tool. (Note that this technique is not quite as foolproof as the other approaches since ISPs could be blocking outbound packets from their customers, which the other approaches would detect but log-watching would not.)
After completing the experiments above, remember to return your system to its previous tight security and verify that everything is safe again by re-running any of our tests.