ZyXEL Communications 1001H manual Introduction, Advantages of NAT, How NAT works

P100IH ISDN Router

Chapter 4: NAT

4.1Introduction

NAT (Network Address Translation - NAT, RFC 1631) is the translation of an Internet Protocol address used within one network to a different IP address known within another network. One network is designated the inside network and the other is the outside. Typically, a company maps its local inside network addresses to one or more global outside IP addresses and “unmaps” the global IP addresses on incoming packets back into local IP addresses. The IP addresses for the NAT can be either fixed or dynamically assigned by the ISP. In addition, you can designate servers, e.g., 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 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).

4.1.1Advantages of NAT

zNAT is a cost-effective solution to access the Internet or other remote TCP/IP networks as NAT conserves on the number of global IP addresses that a company needs in its communication with the outside world.

zNAT supports popular Internet applications such as MS traceroute, CuSeeMe, IRC, RealAudio, VDOLive, Quake and PPTP with no extra configuration needed.

zNAT supports servers, including multiple servers of the same type, to be accessible to the outside world.

zNAT can provide firewall protection if you do not specify a server (for Many-to-One and Many-to- Many Overload mapping) and all incoming inquiries will be filtered out by your Prestige.

zUDP and TCP packets can be routed. In addition, partial ICMP, including echo and traceroute, is supported.

4.1.2How NAT works

Each packet consists of two addresses – a source address and a destination address. For outgoing packets, the ILA is the source address on the LAN, and the IGA 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. The term “Inside” refers to the set of networks that are subject to translation. Network Address Translation operates by mapping 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

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