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Let us illustrate PAT functioning:
There is an internal network 191.167.0 and a router with a MAC address 193.200.150.5. A host
from the internal network with an address 191.167.0.10 and TCP source port 1243 addresses
web-server 205.131.1.1. While passing through the NAT interface, the outcoming packet will
have the following changes: in the IP header, the source address is changed and the source port
in the TCP header is changed from 1243 into, for example, 62300. The following change is then
recorded in the Nat table:
Internal IP Port local NAT port
191.167.0.10 1243 61300
Thus, when the web-server’s reply is received, the Nat table will be updated and the packet
addressed to port 61300, will be corrected: the internal address will be then in the IP header, and
in the TCP header – port 1243, now acting as a destination port.
Incoming connections are impossible with masquerading, since even when a host has an entry in
the masquerading table of the NAT device this entry is only valid for the connection being active.
Version: 1.0 Page. 30 of 95