ZyXEL Communications P-2302HWUDL-P1 Series User Manual

Chapter 9 SIP

P-2302HWUDL-P1 Series User’s Guide

Figure 76   SIP Redirect Server

A SIP register server maintains a database of SIP identity-to-IP address (or domain name) 
mapping. The register server checks your user name and password when you register. 

When you make a VoIP call using SIP, the RTP (Real time Transport Protocol) is used to 
handle voice data transfer. See RFC 1889 for details on RTP.

The ZyXEL Device must register its public IP address with a SIP register server. If there is a 
NAT router between the ZyXEL Device and the SIP register server, the ZyXEL Device 
probably has a private IP address. The ZyXEL Device lists its IP address in the SIP message 
that it sends to the SIP register server. NAT does not translate this IP address in the SIP 
message. The SIP register server gets the ZyXEL Device’s IP address from inside the SIP 
message and maps it to your SIP identity. If the ZyXEL Device has a private IP address listed 
in the SIP message, the SIP server cannot map it to your SIP identity. See 

 for more information about NAT.

Use a SIP ALG (Application Layer Gateway), Use NAT, STUN, or outbound proxy to allow 
the ZyXEL Device to list its public IP address in the SIP messages.

See