ZyXEL ZyWALL 5 User Guide - Page 449
NAT Application, Port Restricted Cone NAT
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Chapter 22 Network Address Translation (NAT) NAT Application The following figure illustrates a possible NAT application, where three inside LANs (logical LANs using IP Alias) behind the ZyWALL can communicate with three distinct WAN networks. More examples follow at the end of this chapter. Figure 260 NAT Application With IP Alias Port Restricted Cone NAT ZyWALL ZyNOS version 4.00 and later uses port restricted cone NAT. Port restricted cone NAT 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 ZyWALL 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. 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 server with IP address 1 and port A sends packets to IP address 3, port C and IP address 4, port D. The ZyWALL changes the server'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 ZyWALL will perform NAT on them and send them to the server 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. ZyWALL 5/35/70 Series User's Guide 449
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