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DI-206 ISDN Remote Router
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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-206 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.
192.168.100.2
192.168.100.3
192.168.100.5
192.168.100.4
192.168.100.1
Single
Global
IP Address
176.220.22.1
LAN
LAN
Port
Local IP
ISDN
Port
WAN
Router
NAT
Translator
Please note that in the above diagram, the Gateway IP address settings for the local
PC’s needs to be set to 192.168.100.1, the LAN IP address of the router.
NAT manipulates the IP addresses in packet headers on a one-to-one basis. An
outgoing data packet (a packet originating from a computer on the local LAN and
destined for a computer outside the private network) will have its IP address
translated as shown below.