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) |
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80 | 6000 | 192.168.100.2 |
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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
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