How To Make iChat AV Work Across Firewalls and NATs

If you want to use the video and audio conferencing facilities of your personal computer, you need to be aware that you may encounter problems in making a successful audio or video connection, when one of the participating end points is behind a firewall or a router with NAT.

Macs and iChat AV are not immune to this.
While most of you are Windows users, much of the higher level info here below applies to you as well.

Network Address Translation (NAT)
Some Internet service providers (ISPs) and home networking routers use a technology called network address translation (NAT) to share an Internet connection. Though this often interferes with video and audio connections in other conferencing applications, iChat AV uses an innovative approach to establish a direct audio and video connection even on networks that feature NAT. In fact, iChat AV works fine with many popular household routers in their out-of-box configurations.

About firewalls
Frequently used by corporations and educational institutions for increased security, firewalls work by blocking certain Internet traffic from entering or leaving a network. Mac OS X also includes a personal firewall that you can find in the Sharing preference pane.

Internet traffic moves through a firewall based on service identification numbers that are known as ports. Certain ports must be open for iChat AV to work. Network administrators typically open a minimal amount of network ports, allowing the traffic for approved applications to enter and leave the network while blocking other network traffic.

Find out which ports need to be enabled in order for your Mac iChat AV to work properly. Read the full technical support paper here:
iChat AV: How to Use With a Firewall or NAT Router

See also:
For firewall issues specific to file transfer, see technical document 107476, "iChat: Cannot Send or Receive a File When Firewall Is Active".