Справочник Пользователя для ZyXEL Communications P-660RU-Tx

Chapter 10 Network Address Translation (NAT)

P-660RU-Tx User’s Guide

Inside/outside denotes where a host is located relative to the P-660RU-Tx, for 
example, the computers of your subscribers are the inside hosts, while the web 
servers on the Internet are the outside hosts. 

Public/local denotes the IP address of a host in a packet as the packet traverses a 
router, for example, the local address refers to the IP address of a host when the 
packet is in the local network, while the public address refers to the IP address of 
the host when the same packet is traveling in the WAN side. 

Note that inside/outside refers to the location of a host, while public/local refers to 
the IP address of a host used in a packet. Thus, an inside local address is the IP 
address of an inside host in a packet when the packet is still in the local network, 
while an inside public address is the IP address of the same inside host when the 
packet is on the WAN side. The following table summarizes this information.

NAT never changes the IP address (either local or public) of an outside host.

In the simplest form, NAT changes the source IP address in a packet received from 
a subscriber (the inside local address) to another (the inside public address) 
before forwarding the packet to the WAN side. When the response comes back, 
NAT translates the destination address (the inside public address) back to the 
inside local address before forwarding it to the original inside host. Note that the 
IP address (either local or public) of an outside host is never changed.

The public IP addresses for the inside hosts can be either static or dynamically 
assigned by the ISP. In addition, you can designate servers, for example, a web 
server and a telnet server, on your local network and make them accessible to the 
outside world. If you do not define any servers, NAT offers the additional benefit of 
firewall protection. With no servers defined, your P-660RU-Tx filters out all 
incoming inquiries, thus preventing intruders from probing your network.

Table 26   NAT Definitions

Inside

This refers to the host on the LAN.

Outside

This refers to the host on the WAN.

Local

This refers to the packet address (source or destination) as the packet travels 
on the LAN.

Public

This refers to the packet address (source or destination) as the packet travels 
on the WAN.