BASIC: What's a client? What is a Dedicated and Listen Server - what is what?
Clients, Listening and Dedicated Servers, Introduction
What is a (dedicated) Server, Client?! Hmm? Which ports and what protocols does it use?
A server is a machine which hosts a game. This means that the server machine takes over the task of managing all the players, who should see what and so on. The server is the center of all the action (the authority) and has access to all game-relevant data. A dedicated server is a stand-alone machine from which you don't play, but only host the game. If you play at the same engine where you host the server you say it is a listening server.
A client is a machine which connects to a server. A client is (for Unreal) a machine you play from. The server you connect to chooses what you see and usually all data the client gets is read-only (for example the scorelist). The client (unless logged in as administrator) has only control over one player.
Unreal servers use the UDP/IP protocol for game control on any port (by default, the gaming port is 7777). UDP is also used for the server query (that is when a client asks who is online playing at the moment from outside before it has connected into the game). The port for the query is ALWAYS the gameport+1 (7778 by default). It is a common misunderstanding that the UBrowser shows the wrong port. To manually connect to a server you need to specify the port-1 (which is true), but the port is not wrong, it is just not the gameport ;-P Unreal uses TCP/IP (??) to connect to master servers to inform them about their presense.
A few more words about listening servers Listening servers are generally less secure, perform worse and crash more often than dedicated servers. The crash issues are related mostly since server and client engine are one and the same, and in most times the client part crashes taking the server down along. If you can, host a dedicated server, not a listening server!