Outside

Packet 1 Source address: 131.84.93.5 Destination address: 206.35.91.10 Destination port = 25

Packet 2 Source address: 131.84.93.5 Destination address: 192.168.50.2

Packet 2 Source address: 131.84.93.5 Destination address: 192.168.50.2

Packet 2 Source address: 131.84.93.5 Destination address: 206.35.91.10 Destination port = 80

In Figure 4-10, the enterprise has a mail server at the local address 192.168.50.1 and an HTTP server at the local address 192.168.50.2. Both servers have a global address of 206.35.91.10. When a host from the outside sends a packet to the inside, the NAT examines the destination port in addition to the destination address. In Figure 4-10, a host has sent a packet to 206.35.91.10 with a destination port of 25, indicating mail. The NAT translates this packet's destination address to the mail server's, 192.168.50.1. A second packet from the same host has a destination port of 80, indicating HTTP. The NAT translates this packet's destination address to the Web server's, 192.168.50.2.

NAT Issues

Although the general applications of NAT presented so far are straightforward, the underlying functions of NAT can be less so, because of the following two factors:

• The general processing of IP and TCP headers

• The nature of some specific protocols and applications

Changing the content of an IP address or TCP port can change the meaning of some of the other fields, especially the checksum. And many protocols and applications carry the IP address or information based on the IP address within their data fields. Changing an IP address in the header could change the meaning of the encapsulated data, possibly breaking the application. This section examines the most common issues surrounding the operation of NAT.

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