ZyXEL Communications P-870HW-I User Manual

P-870HW-I1 User’s Guide

Appendix D NAT

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The following table summarizes how these NAT types handle outgoing and incoming packets. 
Read the following sections for more details and examples.

The examples in these NAT type sections describe NAT translation between internal (private) 
and external (public) IP addresses. 

In full cone NAT, the NAT router maps all outgoing packets from an internal IP address and 
port to a single IP address and port on the external network.  The NAT router also maps 
packets coming to that external IP address and port to the internal IP address and port.

In the following example, the NAT router maps the source address of all packets sent from the 
ZyXEL Device’s internal IP address 1 and port A to IP address 2 and port B on the external 
network. The NAT router also performs NAT on all incoming packets sent to IP address 2 and 
port B and sends them to IP address 1, port A.

Table 172   NAT Types

Incoming 

Packets

Any external host 

can send packets to 

the mapped 

external IP address 

and port.

Only external hosts 

with an IP address 

to which the internal 

host has already 

sent a packet can 

send packets to the 

mapped external IP 

address and port.

Only external hosts 

with an IP address 

and port to which 

the internal host 

has already sent a 

packet can send 

packets to the 

mapped external IP 

address and port.

A host on the external 

network can only send 

packets to the specific 

mapped external IP 

address and port that the 

NAT router used in 

sending a packet to the 

external host’s IP address 

and port. 

Outgoing 

Packets

The NAT router maps the internal IP address and port of all 

outgoing packets to a single IP address and port on the 

external network. 

The NAT router maps the 

internal IP address and 

port of each outgoing 

packet  to a different 

external IP address and 

port for each different 

destination IP address 

and port.