Figure 34 Port Restricted Cone NAT Example
7.3.2.4 Symmetric NAT
The full, restricted and port restricted cone NAT types use the same mapping for an outgoing packet’s source address regardless of the destination IP address and port. In symmetric NAT, the mapping of an outgoing packet’s source address to a source address in another network is different for each different destination IP address and port.
In the following example, the NAT router maps the Prestige’s source address IP address 1 and port A to IP address 2 and port B on the external network for packets sent to IP address 3 and port B. The NAT router uses a different mapping (IP address 2 and port M) when the Prestige sends packets to IP address 4 and port D.
A host on the external network (IP address 3 and port C for example) can only send packets to the internal host via the external IP address and port that the NAT router used in sending a packet to the external host’s IP address and port. So in the example, only 3, C is allowed to send packets to 2, B and only 4, D is allowed to send packets to 2, M.
Chapter 7 Introduction to VoIP | 98 |