A gatekeeper is an H.323 entity on the network that provides services such as address translation and network access control for H.323 terminals, gateways, and MCUs. Also, they can provide other services such as bandwidth management, accounting, and dial plans that you can centralize in order to provide salability.
Gatekeepers are logically separated from H.323 endpoints such as terminals and gateways. They are optional in an H.323 network. But if a gatekeeper is present, endpoints must use the services provided.
http://www.cisco.com/en/US/tech/tk1077/tech_configuration_examples_list.html
http://www.cisco.com/en/US/tech/tk1077/technologies_configuration_example09186a00800a9a56.shtml
