If you are referring to the firewall built into the operating system, that's only half of the picture. Your Internet router (commonly but incorrectly referred to as a modem) is likely to have Firewall-like capabilities. More accurately, the router needs to know which computer to forward network traffic on certain ports to.
For the sake of making this as clear as possible, let's say you have DSL service through the telephone company and they have provided you with an ActionTec DSL router and it is configured to run DHCP. When your computer boots up, it asks the DSL router for an internal IP address to uniquely identify it on your local network. The DSL modem responds to your computer with an assignment of an available internal IP address. We'll use 192.168.0.3 in this example.
When your computer sends out requests for various IP services, such as a web page, the router knows that the request is coming from 192.168.0.3, makes a note of it and forwards the request out to the Internet. When the response comes back, the DSL router knows that the response should be forwarded on to 192.168.0.3 on the local network.
When it comes to video conferencing with iChat, it gets a little more complicated because of the two-way nature of the video conference. What's required here is to configure the DSL modem to pass data coming in on certain port numbers to your computer.
To accomplish this, you need to access the configuration interface for the DSL router by entering your DSL router's IP address into your browser's address bar and pressing enter. To determine your router's IP address, open System Preferences > Network, select your network Interface and in the TCP/IP tab you'll see the router address listed there. It typically ends in '1'. So in our example, the router address would be 192.168.0.1. If you enter your router's IP into your browser and press enter, you'll get the admin interface for the DSL router.
You'll likely be asked for the admin username and password. If you've never set the password, it's likely blank or "password" or "admin" or "1234". Check the documentation that came with your router if you're unable to figure it out.
Once you're in, you can configure the DSL router. There's no exact standard terminology used by the various brands of routers out there, so I can't tell you exactly where to go to configure it properly. For example, in ActionTec routers, the section you are looking for is under Advanced Setup > Port Forwarding. Under LinkSys, DLink, Netgear, routers, it may be called "Services" or "Applications", "Ports", etc.
The key thing to look for is a form where you can add a beginning/starting port number, an ending port number and an IP address field (sometimes referred to as the "destination").
Next you'll refer to the article I pointed out in my previous post above (
http://docs.info.apple.com/article.html?artnum=93208) to see what port numbers you'll be forwarding and add those entries into your router configuration making sure that the IP you are entering is the same IP as the computer that you are trying to use for video conferencing with iChat.
Now before you go and get all excited, there's one other important step to consider. With DHCP, if you reboot your computer or the power goes out or you unplug your DSL router to reset it, etc ... your computer is going to probably get a different IP assigned to it by the DSL modem and, as a result, you'll have to go in and update all of your port forwarding entries to match your new IP.
The best way to circumvent this inconvenience is to write down your current TCP/IP settings for your computer then switch the TCP/IP "Configure IPv4" menu from "Using DHCP" to "Manually". You'll enter everything the same as was listed under DCHP -- same subnet mask, same router address, same DNS (if any, often it's blank) and same IP with one exception -- change the last digit to a high number like 200 or 250 -- you cannot go any higher than 253 (or maybe 254). So in our example, I would set my IP address to 192.168.0.250, my subnet to 255.255.255.0 and my router to 192.168.0.1. Then click apply and test Safari to make sure I can still pull up web sites. If I'm not able to, I'll check the DSL router configuration to see if it has a smaller DCHP range than the standard one. If it's still not able to find web sites, you may need to enter DNS servers into your TCP/IP configuration. To find out which ones you should use, log back into your router configuration and look for DNS server addresses which are often listed on the "Status" page. Make sure you add only one address per line in your DNS field on your computer -- press return after the first one then enter the second.
Then once my computer's IP is set and working, it will no longer change if the power goes out or I reboot it or whatever. I've chosen a higher number because when a DHCP server starts dolling out IP addresses, it starts with 2, then 3, then 4 and so on. The higher number virtually ensures that I won't run into a conflict with other computers/devices on the network.
Whew! Any questions?
NOTE: Skype does video conferencing over port 80 (the standard web port), so you don't have to deal with all of this non-sense.
