Nat And Sip; Figure 34 Symmetric Nat: Outgoing; Figure 35 Symmetric Nat: Incoming - ZyXEL Communications Prestige 2302R User Manual

Prestige 2302R Series User's Guide

Figure 34 Symmetric NAT: Outgoing

In the following figure, Y can send packets to a.b.c.d:1234 (see 1 in the figure) since that
mapping was used when X sent a packet to Y. Y cannot send packets to a.b.c.d:5678 (2 in the
figure).
Z can send packets to IP address a.b.c.d:45678 (2 in the figure) since that mapping was used
when X sent a packet to Z. Z cannot send packets to a.b.c.d:1234 (1 in the figure).

Figure 35 Symmetric NAT: Incoming

6.4 NAT and SIP

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) or the Use NAT, STUN, and outbound proxy
features allow the Prestige to list its public IP address in the SIP messages.
88 Chapter 6 Introduction to VoIP