Prestige 2000W User’s Guide
Chapter 12 User Web Configurator Screens
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12.11.1 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.
12.11.2 Fake WAN 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 this port number to the
Prestige.
12.11.3 STUN
STUN (Simple Traversal of User Datagram Protocol (UDP) through Network Address
Translators) allows the Prestige to find the presence and types of NAT routers and/or firewalls
between it and the public Internet. STUN also allows the Prestige to find the public IP address
that NAT assigned, so the Prestige can embed it in the SIP data stream. STUN does not work
with symmetric NAT routers (see
) or firewalls. See RFC 3489
for details on STUN.
The following figure shows how STUN works.
1
The Prestige (A) sends SIP packets to the STUN server (B).
2
The STUN server (B) finds the public IP address and port number that the NAT router
used on the Prestige’s SIP packets and sends them to the Prestige.
3
The Prestige uses the public IP address and port number in the SIP packets that it sends to
the SIP server (C).
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