D-Link DI-308 - Manual (Page 65)

DI-308 ISDN Remote Router
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NAT Configuration
Network Address Translation (NAT) is a routing protocol that allows your network
to become a private network that is isolated from, yet connected to the Internet. It
does this by changing the IP address of packets from a global IP address usable on
the Internet to a local IP address usable on your private network (but not on the
Internet) and vice-versa.
NAT has two major benefits. First, NAT allows many users to access the Internet
using a small number or even a single global IP address. This can greatly reduce the
costs associated with Internet access and also helps alleviate the current shortage of
Internet IP addresses. Secondly, the NAT process creates a firewall which hides your
local network from Internet users, providing a degree of security to your Internet
connection.
To be successfully implemented, NAT should be used only when the majority of
network traffic remains on the local network. In cases where a large percentage of
network traffic is destined for the Internet, NAT can adversely affect the speed and
performance of your Internet connection. Also, your network servers such as ftp
servers, web servers or mail servers will probably need to be assigned static NAT IP
addresses so their IP addresses remain consistent. This issue will be further
discussed later.
Network Address Port Translation (NAPT) is a subset of NAT where many local IP
addresses and their TCP/UDP port numbers are translated to a single global IP
address and it’s TCP/UDP port number. In this document, the term NAT will refer to
both NAT and NAPT unless otherwise stated.
NAT can work in conjunction with DHCP. Thus, if both are enabled and properly
configured, the DHCP server in the DI-308 will assign local IP addresses to
computers on your network.
How NAT Works
In the most common NAT configuration, your network uses local IP addresses that
are not valid on the Internet. Internet (global) IP addresses are unique, with no two
devices have the same IP address. The local IP addresses can be freely assigned to
computers on your network by your network administrator (within guidelines
defined later in this chapter and in Appendix B,”IP Concepts”). This can be done
manually or by using DHCP. The ISDN port on the router is assigned a globally
unique IP Address that IS valid on the Internet, since it will be sending and receiving
data directly to the Internet and is therefore part of it. Please study the example
diagram below carefully.