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Prestige 2002 Series User’s Guide
Chapter 5 Introduction to VoIP 45
Figure 18 Port Restricted Cone NAT
5.3.2.4 Symmetric NAT
Symmetric NAT is different from the cone NAT types in that a specific NAT mapping of a
private IP address and port to a public IP address and port depends on the packet’s destination
IP address. In the following figure, if A sends a packet from IP address 10.0.0.3 and port 80 to
B, it may be mapped as IP address a.b.c.d and port 45678, but if A sends from the same IP
address and port to a different IP address, it is mapped differently (IP address a.b.c.d and port
12345).
Figure 19 Symmetric NAT
Computer B can only respond to its mapping (IP address a.b.c.d and port 45678) and computer
C can only respond to its mapping (IP address a.b.c.d and port 12345). If either computer B or
C tries to send to the other’s mapped IP address and port, the NAT router will drop the
packets. Like restricted cone NAT, a host on the Internet can only send a packet to the private
IP address and port if the private IP address and port has previously sent a packet to that host.