Table of Contents
Advertisement
7-2 NAT
NAT never changes the IP address (either local or global) of an outside host.
7.1.2
What NAT Does
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 global address) before forwarding the packet to the
WAN side. When the response comes back, NAT translates the destination address (the inside
global address) back to the inside local address before forwarding it to the original inside host.
Note that the IP address (either local or global) of an outside host is never changed.
The global 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. You can make designated servers on the
LAN accessible to the outside world. If you do not define any servers (for Many-to-One and Many-
to-Many Overload mapping), NAT offers the additional benefit of firewall protection. With no
servers defined, your Contivity 221 filters out all incoming inquiries, thus preventing intruders
from probing your network. For more information on IP address translation, refer to RFC 1631,
The IP Network Address Translator (NAT).
7.1.3
How NAT Works
Each packet has two addresses – a source address and a destination address. For outgoing packets,
the ILA (Inside Local Address) is the source address on the LAN, and the IGA (Inside Global
Address) is the source address on the WAN. For incoming packets, the ILA is the destination
address on the LAN, and the IGA is the destination address on the WAN. NAT maps private (local)
IP addresses to globally unique ones required for communication with hosts on other networks. It
replaces the original IP source address (and TCP or UDP source port numbers for Many-to-One
and Many-to-Many Overload NAT mapping) in each packet and then forwards it to the Internet.
The Contivity 221 keeps track of the original addresses and port numbers so incoming reply
packets can have their original values restored. The following figure illustrates this.
317517-A Rev 00
Advertisement
Table of Contents
