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Prestige 791R G.SHDSL Router
11-2 NAT
NAT never changes the IP address (either local or global) of an outside host.
11.1.2 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. Although you can make designated servers on the LAN
accessible to the outside world, it is strongly recommended that you attach those servers to the DMZ port
instead. If you do not define any servers (for Many-to-One and Many-to-Many Overload mapping – see
Table 11-2), NAT offers the additional benefit of firewall protection. With no servers defined, your
Prestige 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).
11.1.3 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 Prestige keeps track of the original addresses and port
numbers so incoming reply packets can have their original values restored. The following figure illustrates
this.