Router User Manual

4 NAT troubleshooting
Nortel Secure Router 8000 Series
Troubleshooting - VAS
4-2 Nortel Networks Inc. Issue 01.01 (30 March 2009)
4.1 NAT
4.1.1 NAT attributes
Network Address Translation (NAT) allow hosts in a private network to share IP addresses for
Internet access. With the rapid growth in Internet scales, the number of IPv4 addresses
becomes insufficient. NAT is thus introduced to save IP addresses.
Using NAT, multiple users can access the Internet through a small number of IPv4 addresses
at the same time. The access to the Internet is transparent for most application programs. You
need not configure on clients.
Each host has only a private IP address. The private IP addresses are reserved by the Internet
Assigned Numbers Authority (IANA) for private networks and used by hosts to access each
other. When a host wants to access the Internet, its private IP address is replaced by a legal IP
address on the egress of the private network. The mapping between a private IP address and a
legal IP address is recorded so that the legal IP address of a returned packet can be changed
back into the private IP address. The mapping remains till the end of the access.
Figure 4-1 NAT principles
Internal
network
External
network
162.105.178.65
162.105.178.66
162.105.178.67
162.105.178.68
......
Address pool
10.1.1.200
211.100.7.34
162.105.178.65
211.100.7.34
10.1.1.200
211.100.7.34
162.105.178.65
211.100.7.34
10.1.1.200 211.100.7.34
NAT Server