Networks

I'll answer one of your questions, "how does an incoming connection know where to go?" A: the router assigns the client computer a IP address from a pool. This address is then used when the client requests information from a site. The web site sending the information then sends packets of that information with the clients IP address [and other particular info] in what I'll term a "header". The router reads the incoming header information [along with some other information] and directs all packets to the correct client via the built in switch.

Here's a very brief and non detailed synopsis. Your computer uses ports to make connections [like port 80 is used for connecting to web sites or port 21 for file transfer]. If port 80 is blocked, you can't connect to web sites that need to pass traffic on port 80. Software programs like Windows update server also use ports for internet and internal network traffic. There are about 65000 ports that programs can use to conduct traffic. There is a group of engineers and others that handles defining what ports do what[type in well know ports as a search parameter in a browser and take a peek at some articles.
Firewalls typically block all ports until you specifically allow traffic [continued with comments section].

Since you don't provide any information on what programs your using or want to use for video conferencing [interactive internet video] it is difficult to provide a definitive answer.

Below is an article from Microsoft which can give you an overview of their net meeting video conferencing. If you provide specifics, a more definitive answer can be given.


http://www.microsoft.com/technet/treeview/default.asp?url=/technet/prodtechnol/netmting/reskit/netmtg3/part2/chapter4.asp