every conferencing system that registers with the gatekeeper provides its user number (also called an e.164 number) and a user name (also called an h.323 alias). the gatekeeper keeps a database so it can map between the ip address, the user number and the user name of each system. therefore, you can dial these numbers or names instead of ip addresses.
user numbers and names are static. they do not change, as long as a system remains registered with the same gatekeeper. ip addresses are usually dynamic; they may change without notification. ask your system administrator what your user number and name are.
gatekeepers also provide access control, which means that you cannot communicate with non-registered systems. fortunately, gatekeeper zones are not limited to lan segments; they can connect international wans. if you want to call someone who is registered with another gatekeeper, ask the system administrators to set the gatekeepers as neighbors. neighboring gatekeepers share databases; their endpoints can contact each other. make sure that your conferencing contacts are registered either with your gatekeeper, or with one that neighbors it.