3Com REMOTE ACCESS SYSTEM 1500, Management Manual

Page: 257 / 300

Overview
255
Figure 20 depicts static NAT.
Figure 20 Static NAT
Port Address
Translation
PAT translates Internet Protocol (IP) addresses and User Datagram
Protocol (UDP) or Transmission Control Protocol (TCP) source port
numbers.
For example, assume your ISP assigns you a public IP address of 200.1.1.1
and a user on your private network (with an IP address of 192.168.111.1,
for example) attempts to access the public network. The following
happens:
■
The RAS 1500, when it receives the “outbound” packet, uses PAT to
translate the private source address and source port number. For
example, 192.168.111.1, port 4444 is translated to the ISP-assigned
public IP address, 200.1.1.1, port 5001. The RAS 1500 maintains a
dynamic PAT mapping for this translation.
■
Then, when an “inbound” packet addressed to 200.1.1.1, port 5001
arrives at the RAS 1500 from the public network, the RAS 1500 uses
the dynamic PAT mapping to reverse the translation (from 200.1.1.1,
port 5001 to 192.168.111.1, port 4444), and the packet is routed to
the correct user on the private network.
■
The next user is assigned the same public IP address and the next free
port number. For example, 200.1.1.1, port 5002. When the
connection for a user ends, the port number becomes available for
other users.
Public
192.168.111.1
Private
200.1.1.1
192.168.111.4
192.168.111.3
192.168.111.2
200.1.1.4
200.1.1.3
200.1.1.2
When a user attempts to access the public
network, the private address of the user is
assigned the public IP address defined in
the table.