Table of Contents
Advertisement
7-2 Network Address Translation (NAT) Screens
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. If you do not define any servers (for Many-to-One and Many-to-
Many Overload mapping – see Table 7-2), NAT offers the additional benefit of
firewall protection. With no servers defined, your Contivity 251 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 251 keeps track of the original
addresses and port numbers so incoming reply packets can have their original
values restored. The following figure illustrates this.
317516-A Rev 00
Advertisement
Table of Contents
