ZyXEL Communications P-870HW-I Series, User Manual, Page: 387 / 438

P-870HW-I1 User’s Guide
Appendix D NAT 387
Figure 267 Restricted Cone NAT Example
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 ZyXEL Device 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 ZyXEL Device’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 ZyXEL Device 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 .