CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

- Port Address Translation (PAT) was introduced to overcome the limitations of static and
dynamic NAT methods. It performs many-to-one mapping between multiple unregistered
private IP addresses and a single registered public IP address by using transport layer ports.
PAT is also known as Overload NAT or Dynamic NAT with Overloading.
172.16.1.1
172.16.1.2
172.16.1.3
172.16.1.1
172.16.1.1, Port 1000
172.16.1.2, Port 2000
172.16.1.3, Port 3000
Internet
Internet
Figure 17-5: TCP Connections from Multiple Hosts and a Single Host
- Figure 17-5 shows that a server doesn’t care how many clients are connected to it at a time.
The server is able to differentiate connections to it by the combination of client IP addresses
(network layer) and port numbers (transport layer).
- PAT utilizes the advantage of the TCP/IP multiplexing feature, where connections are unique
with the combination of IP addresses and port numbers. A NAT router would have a NAT table
entry for storing every unique combination of IP addresses and port numbers when hosts in
private network traverse through it to the Internet. The port numbers assigned are random and
will not be from the well-known port number range – 1 to 1023. These entries are used for
reverse-translation upon reply packets from the Internet back to the hosts in private network.
- PAT does not only translate IP addresses, it also translates port numbers. Theoretically, 65536
connections to the hosts on the Internet can be made with a single registered public IP address.
127
200.1.1.1, Port 80
200.1.1.1, Port 80
200.1.1.1, Port 80
Figure 17-5A: 3 TCP connections from 3 different hosts
172.16.1.1, Port 1000
172.16.1.1, Port 1001
172.16.1.1, Port 1002
200.1.1.1, Port 80
200.1.1.1, Port 80
200.1.1.1, Port 80
Figure 17-5B: 3 TCP connections from 1 single host
200.1.1.1
200.1.1.1
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com