ZyXEL Communications Broadband Security Gateway P-312 - Manual (Page 75)

P312 Broadband Security Gateway
6-2 NAT
them accessible to the outside world. If you do not define any servers (for Many-to-One and Many-to-Many
Overload mapping – see below), NAT offers the additional benefit of firewall protection. If no server is
defined in these cases, all incoming inquiries will be filtered out by your Prestige, thus preventing intruders
from probing your network. For more information on IP address translation, refer to RFC 1631, The IP
Network Address Translator (NAT).
6.1.3 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 diagram
illustrates this.
Figure 6-1 How NAT Works
6.1.4 NAT Mapping Types
NAT supports five types of IP/port mapping. They are:
1. One to One: In One-to-One mode, the Prestige maps one local IP address to one global IP address.