Receiving files, audio, and video from others requires your computer to receive data from the Internet. By default, Messenger will instruct the Windows Firewall to permit this data to pass. Only if you've deliberately unchecked Windows Messenger in the Windows Firewall Exceptions list will these services fail to work. If your network has some other sort of firewall set up, you'll need to be sure that certain network port numbers are open in order to use Messenger services. Table 12.2 lists these ports.
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All of this applies also to the people you want to chat with, if they also use a shared Internet connection.
Now, for the terminally curious, here's the technical background.
The Internet routes data back and forth between computers identified by a number called an IP address. When several computers share a single Internet connection, all of the communications share a single IP address, and the router takes care of directing incoming data to the appropriate computer. It's a lot like the telephone switchboard of a large company: There's one public telephone number, and the operator takes care of routing incoming phone calls to the internal extensions. This is what Network Address Translation is all about, whether it's performed by a hardware connection sharing router device or Windows's Internet Connection Sharing service.
When an Internet connection is initiated by your computer and directed to a computer elsewhere on the Internet, NAT sees your outgoing data and expects data to come back; it knows to send the returned data to your computer. Basic text chat works fine with NAT, because each computer in the conversation makes a connection to an outside chat server run by Microsoft (or by your company, if you're using a corporate network). NAT sees this connection as it's made, and the chat server sends text back and forth through the established data connection. Likewise, if you use File Transfer to send a file to a friend, NAT sees your attempt to contact your friend's computer and tracks the conversation.
Unfortunately, if another computer spontaneously attempts to establish a connection directly to yours, the NAT "operator" isn't expecting the call, and it won't know to whom to direct the incoming data. Sending a file via Messenger's File Transfer option involves an incoming connection initiated by the remote computer directly to yours (to pick up the file), so without special preparations, without special help, it won't work over a shared Internet connection. Likewise, audio and video chat, the whiteboard, and application sharing services all have problems with NAT.
There are two ways they can work. You could manually configure your sharing router to forward incoming connections that use certain "port" numbers to your computer, by entering the port number and your IP address on the router's Applications or Forwarding setup screen.
However, if your router or connection sharing service supports the Universal Plug and Play (UPnP) protocol, it's much easier. Windows Messenger will automatically tell the router to expect an incoming connection, and the router will know to forward it to your computer. UPnP is built into the Windows Internet Connection Sharing service, which is why Messenger works with ICS right out of the box. If you use a hardware sharing router, check to see if it supports UPnP. Nowadays, most routers do. However, you may have to upgrade the router's firmware to get UPnP support; check the manufacturer's Web site for update information.
Messenger's audio and video chat, whiteboard, and application sharing have an additional problem. These services use the Session Initiation Protocol (SIP), in which each computer in the conversation tells the other its IP address in a way that NAT normally can't see and translate. The result is that the two computers attempt to send data to IP addresses that don't exist out on the Internet, and the chat session never gets off the ground. If your connection sharing service or router supports UPnP, Messenger gets around this problem by asking the router for its external, public IP address, and uses this inside the SIP message. If your router doesn't support UPnP, in addition to forwarding the required connection ports, the router must also be "SIP-aware"; that is, it has to know to look specifically for SIP data and to modify it as it it's passed through. SIP support is much less common than UPnP in inexpensive connection sharing routers, so it's unlikely that voice and video will work over a manually forwarded connection. Corporate network routers, on the other hand, probably can be configured to work with SIP, or will have a private SIP server on the network.
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If there are two NAT routers in between your computer and the Internet, UPnP will not save the day for you, as Messenger can negotiate with the first router, but not the second. This might be the case when, for example, someone shares her Internet connection with a neighbor, and the neighbor uses his own router. (I've actually seen this in friends' apartments; one person buys DSL service and they snake network cables out the windows and all over the rest of building.) In this case, you can probably just forget about voice, video, and the rest.
Finally, some technical data. While I highly recommend using UPnP, if you have the masochistic urge to set up manual port forwarding, Table 12.2 lists the ports used by Messenger services. If your network uses a firewall, you'll also have to be sure these ports are open in order to use the associated Messenger services.
You can read more about this topic by searching Microsoft's website for the article "Windows Messenger 5.0 in Windows XP: Working with Firewalls and Network Address Translation Devices."
