TW-H6W1IR ISDN Remote Router
Configuration and Management 83
made. The router translates the IP port number to a unique (global) IP
port number that the router generates itself (outside the range of Well-
Known IP Port Numbers that are used for other network protocols
such as html, telnet, etc.). The global port number and the global IP
address are transcribed onto the packet (replacing the local numbers),
and the packet is sent. The router then adds these values to it’s NAPT
table as shown in the example entry below.
Source Port (local) Source Port (global) Source IP (local)
80 6000 192.168.100.2
The reply packet received by the router will be addressed to the global
IP address and the global port number. The router then searches the
NAPT table to match the IP port number from which it learns the local
destination IP address and port number for the packet. It then translates
the IP address and port number of the packet to their local equivalents
and delivers it to the local host. Since all reply packets received by the
router from the Internet are addressed to the single global IP address
being used, the port number is the decisive parameter telling the router
which local computer to route the packet to.
The above process describes dynamic NAPT. Static NAPT allows you
to map specific global IP port numbers to local IP addresses and port
numbers for certain applications that need to use specific port numbers,
such as web servers (port 80), telnet servers (port 23), etc. Some
Well-Known IP Port Numbers are provided in Appendix C – IP
Protocol and Port Numbers, and a complete list can be found in RFC
1700. Some software applications (such as Microsoft NetMeeting,
