ZyXEL Communications NBG-417N, User Manual, Page: 126 / 279

Chapter 11 NAT
User
’
s Guide
127
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 , NAT offers the additional benefit
of firewall protection. With no servers defined, your NBG4115 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) .
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 in each packet and then forwards it to the
Internet. The NBG4115 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 66 How NAT Works
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