Figure 33 Restricted Cone NAT Example
7.3.2.3 Port Restricted Cone NAT
As in full cone NAT, a port restricted cone NAT router maps all outgoing packets from an internal IP address and port to a single IP address and port on the external network. In the following example, the NAT router maps the source address of all packets sent from internal IP address 1 and port A to IP address 2 and port B on the external network.
The difference from full cone and restricted cone NAT is in how the port restricted cone NAT router handles packets coming in from the external network. A host on the external network (IP address 3 and Port C for example) can only send packets to the internal host if the internal host has already sent a packet to the external host’s IP address and port.
A Prestige with IP address 1 and port A sends packets to IP address 3, port C and IP address 4, port D. The NAT router changes the Prestige’s IP address to 2 and port to B.
Since 1, A has already sent packets to 3, C and 4, D, they can send packets back to 2, B and the NAT router will perform NAT on them and send them to the Prestige at IP address 1, port A.
Packets have not been sent from 1, A to 4, E or 5, so they cannot send packets to 1, A.
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