Microsoft® Windows® XP Networking and Security Inside Out: Also Covers Windows 2000

Why Blocking Ports Isn't Enough

As firewall solutions became popular in the mid-1990s, some Internet application developers found that their software wouldn't work across firewall-guarded networks. Network administrators had done their job well and locked down all but a small handful of ports, such as HTTP port 80. Unfortunately, users behind a firewall now couldn't use services such as RealAudio or AOL Instant Messenger that used other ports. Users who wanted these services could ask their network administrators to open the ports, but users had to know enough to ask, and administrators weren't obligated to say yes.

To circumvent this problem of blocked ports, application developers started using commonly open ports like HTTP port 80 to provide ...