A registration SERVER, also known as a registrar, accepts SIP REGISTER requests and all other requests receive a 501 Not Implemented response. The CONTACT information from the request is then made available to other SIP servers within the same administrative domain, such as proxies and redirect servers. In a registration request, the To HEADER field contains the name of the resource being registered, and the Contact header fields contain the alternative ADDRESSES or aliases. The registration server creates a temporary binding between the Address of Record (AOR) URI in the To and the DEVICE URI in the Contact header.

A registration server, also known as a registrar, accepts SIP REGISTER requests and all other requests receive a 501 Not Implemented response. The contact information from the request is then made available to other SIP servers within the same administrative domain, such as proxies and redirect servers. In a registration request, the To header field contains the name of the resource being registered, and the Contact header fields contain the alternative addresses or aliases. The registration server creates a temporary binding between the Address of Record (AOR) URI in the To and the device URI in the Contact header.