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P-2002 Series User’s Guide
Chapter 5 Introduction to VoIP 53
Figure 22 Symmetric NAT
5.4 NAT and SIP
Some NAT routers are not SIP-friendly and will stop your voice sessions.
The Prestige must register its public IP address with a SIP register server. If there is a NAT
router between the Prestige and the SIP register server, the Prestige probably has a private IP
address. The Prestige 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 Prestige’s IP address from inside the SIP message and maps it to your SIP identity. If the
Prestige has a private IP address listed in the SIP message, the SIP server cannot map it to your
SIP identity.
A SIP ALG (Application Layer Gateway), the fake WAN address on SIP and RTP, STUN, or
outbound proxy features allow the Prestige to use its public IP address in the SIP messages.
5.4.1 SIP ALG
Some NAT routers may include a SIP Application Layer Gateway (ALG). A SIP ALG allows
SIP calls to pass through NAT by examining and translating IP addresses embedded in the data
stream. When the Prestige registers with the SIP register server, the SIP ALG translates the
Prestige’s private IP address inside the SIP data stream to a public IP address. You do not need
to use STUN or an outbound proxy if your Prestige is behind a SIP ALG.
5.4.2 Use NAT Address on SIP and RTP
If you know the NAT router’s public IP address and SIP port number, you can use the Use
NAT feature to manually configure the Prestige to use a them in the SIP messages. This
eliminates the need for STUN or a SIP ALG.
You must also configure the NAT router to forward traffic with the SIP port number to the
Prestige.