3Com Switch 8800 Advanced Software V5 Configuration Guide

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NAT CONFIGURATION
When configuring NAT, go to these sections for information you are interested in:
■ “NAT Overview” on page 985
■ “NAT Configuration Task List” on page 989
■ “Configuring Address Translation” on page 990
■ “Configuring Internal Server” on page 991
■ “Configuring the Binding” on page 992
■ “Configuring NAT Log” on page 993
■ “Configuring User Resource Limit” on page 995
■ “Configuring Connection-limit” on page 996
■ “Displaying and Maintaining NAT” on page 997
■ “NAT Configuration Example” on page 998
■ “Troubleshooting NAT” on page 1003
NAT Overview
Introduction to NAT Network Address Translation (NAT for short) provides a way of translating the IP
address in an IP packet header to another IP address. In practice, NAT is primarily
designed for private network users to access public networks. This way of using a
smaller number of public IP addresses to represent a larger number of private IP
addresses can effectively alleviate the depletion of IP addresses.
n
Private or internal IP addresses refer to IP addresses used in an internal network
whereas public or external IP addresses refer to the globally unique IP addresses
used on the Internet.
According to RFC 1918, three blocks of IP addresses are reserved for private
networks:
■ In Class A: 10.0.0.0 to 10.255.255.255;
■ In Class B: 172.16.0.0 to 172.31.255.255;
■ In Class C: 192.168.0.0 to 192.168.255.255;
The above three ranges of IP addresses are not assigned over the Internet. You can
use these IP addresses in enterprises freely without the need for applying them
from the ISPs or the registration center.
Figure 296 depicts a basic NAT operation: