The thing you need to know is what ports to open up. You have an IP address to the external world, given to you by your ISP, which is sort of like the internet equivalent of a telephone number.

Unlike a phone with a telephone number, though, a computer with an IP address can do many things. Each different network "service" offered by a program on your computer has what is called a "port." It's just a number. Like the program for sending email, which will generally use port 25. Web traffic is port 80. That way, a bunch of traffic can all come to your computer by IP address, and then the port number can be used route the traffic to the right program running on your computer, even if they are all running at the same time.

Most P2P stuff opens connection up TO your machine. This is backward from most stuff you do through a firewall: you connect FROM your machine to some other server. But in P2P you are a server, so programs connect TO you.

Your firewall will not allow this by default except on certain ports; indeed the general point of a firewall is to disallow most incoming connections. What you need to do is figure out which ports WinMX uses, and then tell the firewall to let those ports through. It sounds like in your setup, that either your cable router or your WinXP box is your firewall (or they both might be doing it).

WinMX uses 6699 TCP and 6257 UDP (on top of port numbers, there are also different network protocols that can be used on top of your IP. TCP and UDP are like two different low-level networking languages).

It looks like Shoutcast uses port 8000.

I'm not sure what firewall hardware and/or software you are using, but you would want to open up in-bound connections for those two services with that. So what are you using? Is it your cable modem? And if so, what kind is it? Or is it just the firewall that comes with WinXP?

It's the firewall that came with the Airport extreme and i think the software is on the Mac. altough there's the firewall in XP too, I guess. I'm borderline network illiterate. I need reaaaly simple instructions, step-by-step. i dig the concept, it's the execution that baffles me.

Does this help.
The magic word you're looking for in your Routers configuration is "Virtual Server"

Here's the steps i would take ... (YMMV)
1. Turn off the firewall on your winXP machine. Don't worry too much about this, you're probably behind nats and reasonably safe.

2. Find out your internal ip address. Simply ...
Click Start --> Run.
Type CMD [return]
in the command prompt, type ipconfig /all [return]
make a note of your ip address and gateway address
These will probably be something like 192.168.1.1 (ip address) and 192.168.1.254 (gateway)
type exit.

3. Log onto your router. You'll probably get to this by starting internet explorer and typing in the gateway address (above) in your address bar.

4. Look for the Virtual server settings. Sorry - No idea where they are. A manual may be good for this step.

5. In your virtual server settings, you'll need to enter your ip address and a range of ports. I'd pick ports 8000 --> 8005. This will open your computer up on the defined ports. i.e. when anyone trys to look at the ip address owned by the router on ports 8000+ , they'll be redirected to your computer.

6. Start up your shoutcast server and pray to the baby zombie Jesus that it works.

But... I may be lying... What piece of kit is attached to your cable modem?

The airport extreme is attached to the cable modem.

Then my initial instructions may work.
/crosses fingers.

IE won't log onto my router when I type in the default gateway.

Try looking in the airport extreme configuration utility for anything about virtual servers.
In the case that the extreme acts as a router, then I don't really know the technology, and it's likely that Apple does this differently to everyone else in the world.

A bit of research shows that they use the phrase "port mapping" instead of virtual server. It's the same thing. From the instructions.

Port mapping.If you are running a web, AppleShare, or FTP server on your AirPort
network, other computers need to be able to initiate communication with it. Because
the base station only stores port-to-private IP address table entries for outgoing con-
nection requests, it has no way of directing incoming information to the appropriate
computer.To ensure that requests are routed to your server, you need to establish a
permanent IP address for the server and then use AirPort Admin Utility to provide
inbound port mapping information to the AirPort Extreme Base Station.

and

To set up inbound port mapping:
1 Open AirPort Admin Utility, select your base station, and click Configure.
2 Click Show All Settings.
3 Click the Port Mapping tab.
In the Port Mapping pane, click Add and enter the following information:
• Public Port: The port number other computers will use to access the services
provided by your computer. For example, computers look for web services on
port 80. BUT JON - You'll ENTER 8000 or 8001
• Private IP Address: The manually assigned, private IP address of your computer.
• Private Port: The port that will be used on your computer to provide services. In most
cases, you can use the same number as the public port.
To use port mapping, you must configure TCP/IP manually on the computer that is
running the web, AppleShare, or FTP server.
You can also set up a computer as a default host to establish a permanent IP address
for the computer and provide inbound port mapping information to the AirPort
Extreme Base Station. This is sometimes known as a DMZ and is useful when playing
some network games or videoconferencing.

Do I do that on the Mac or the PC or does it matter?

It shouldn't matter, but I'd do it on you mac for the "ease of use". Remember that you'll need your winxp ip address from ipconfig (above) for your "private ip address"

How come you're not using your mac to do the shoutcasting?

If you've got skype, I'm happy to talk to you for ten mins or so. (Then unfortunately, I have to go to bed)

How come you're not using your mac to do the shoutcasting?

cause I'm a windows guy, and all the music files are on the windows box. I'm beat too. I'll try this all tommorrow and I'll consult with th IT guys at work. Thanks for trying.

I tried to help jonmc with this last night but unfortunately I was already too tired when we started and when we hit network probs I was too far gone to be any use.

Here's what's weird: He's not running a Shoutcast server and shouldn't need to - all he needs to be able to do is send data to a specific port on a specific server which shouldn't require the opening of ports on his firewalls. It should be simple enough however it's clearly getting blocked somewhere.

Is it possible it's his ISP?

Alternatively it could be a flooded connection I suppose - but if that was the case there would be plenty of other evidence, such as webpages timeing out, downloads taking an eternity, etc. Strange.

It's quite annoying there aren't more diagnostic tools to help with this.

Perhaps the ISP blocks streaming media? Some used to do this and it is (just) conceivable that this still happens.