multiplayer servers

I've never played with Flash Communication Servers, it's too expensive, but maybe it can handle Socket Connections...
What you need is a Server Socket - an application, that runs on the server, checking for any new clients logging on it and managing in- and outgoing messages from the clients. You're not allowed to run applications on a web hotel, therefor you must have your own server...
This Server Socket can easiliest be made in JAVA, but any "Stand-alone" script, like C++, can be used, theese are just much harder to learn...

In flash you can use the "XMLSocket" object to connect to a server on a specific port:

socket = new XMLSocket();
socket.connect("domain or IP here", "port - a number");

You can try http://www.electrotank.com/ElectroServer/
The Electroserver can be hosted by webgamehost for a fair price: (cheaper than Comm. server :o))
http://www.webgamehost.com/account_features.html