Chapter 11 Network Address Translation (NAT)
VMG8825-B Series User’s Guide
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NAT never changes the IP address (either local or global) of an outside host.
11.9.2 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. 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 VMG 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)
.
11.9.3 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 VMG keeps track of the original
addresses and port numbers so incoming reply packets can have their original values restored. The
following figure illustrates this.
Figure 96
How NAT Works
DSL