network in a lan house

hello. how is networking done in a lan house... im talking about the internet connectio, they have two main computers, one of them connects to the internet (same cable internet provider i have at home), that computer connects to a hub in wich all the other twenty computers also connect. all computers have the same internet protocol address run microsoft windows xp with no internal or software firewall... in the other day i found out that one of the computers has a door opened, one used by a trojan, it tried it at home, connect to the ip address, but it didnt work, wich is good... but how is this done? if i try to communicate with one of the computers how can that be done? all of them have the same internet protocol number... anyone wants the number, maybe you could get some information.
thanks

I find your post a bit confusing so I might have misunderstood your question.
So it's a shared internet connection?
You can setup the gateway box to route incoming data on a particular port to the box you want it to go to. So it provides NAT or what? (To send the incoming data to the correct box)

i dont even work there... all i know is that they have a normal cable connection, two main computers, a router\hub\switch wich connects those computers and the computers for the clients (20) i dont know if the internet connects to one of the two computers or the hub\switch\router

Ok first off, whenever you connect to the internet your ISP assigns you an IP address. Usually is you have a DSL/Cable line you have a static IP address. (Just one, for the machine that is directly connected to the internet). Now, if you are running your LAN with tcp/ip, all computers on the LAN will have an IP address too. The only IP address the internet can see is the one assigned to the machine that is sharing the internet connection with the LAN, not the computers behind it. (unless they are running a service for some reason..FTP server/ POP3 server, etc)

They will still only see the public address assigned to the router machine. An additional public adress just doesn't appear when you turn on FTP. The router machine with connection sharing opens a connection route and continues to hide the internal address of the FTP server. Unless you physically obtain and then add an additional static address.

thingo, Go to a command prompt and type IPconfig. The machine listed as Gateway will tell you if it's a router another PC or the modem. Or do a trace route to someplace and see what pops up in the route. SephirOth is right the public address will seem the same to all computers from the internet but the computers DO NOT have the same IP address physically. Go around to each one and print out the ipconfig for future reference.