3Com 3C421600A, Management Manual

Page: 255 / 300

16
U
SING
N
ETWORK
A
DDRESS
T
RANSLATION AND
P
ORT
A
DDRESS
T
RANSLATION
This chapter contains the following information:
■
Overview
■
Configuring NAT and PAT
■
Case Studies
Overview
Network Address Translation (NAT) and Port Address Translation (PAT) act
as address translators between public and private networks. They allow
users on a privately addressed network to access the public network.
Use NAT if your Internet Service Provider (ISP) assigns you a public
subnetwork. Use PAT if your ISP assigns you one IP address.
Network Address
Translation
NAT translates IP addresses.
For example, assume your ISP assigns you a public subnetwork
200.1.1.0/28 from which you set aside a pool of public addresses from
200.1.1.1 to 200.1.1.10. When a user on 192.168.111.to 200.1.1.15
and a user on your private network (with an IP address of
192.168.111.1/C on the private network attempts to access a public
host. The following happens:
■
The SuperStack II Remote Access System (RAS) 1500, when it receives
the “outbound” packet, uses NAT to translate the private address,
192.168.111.1, to the first free IP address in the public pool,
200.1.1.1. The RAS 1500 maintains a dynamic NAT mapping for this
translation.
■
Then, when an “inbound” packet addressed to 200.1.1.1 arrives at
the RAS 1500 from the public network, the RAS 1500 uses the
dynamic NAT mapping to reverse the translation (from 200.1.1.1 to