Netopia R5000 User Reference Manual page 257

As you can see, when Workstation A and Workstation B transmit an IP packet to the WWW server on the
Internet, they have unique source IP addresses on the LAN interface but potentially the same source ports,
which in this case is 400. When the Netopia R5000 Series Router receives these packets, the source IP
addresses are substituted with the single globally unique IP address that was acquired on the WAN interface,
which is 200.1.1.40.
Now both IP packets have the exact same source IP address (200.1.1.40) and source ports (400). The Netopia
R5000 Series Router is then able to distinguish between the two IP packets by changing the source TCP or UDP
ports and keeping this information in an internal table. As seen above, the source port for Workstation A has
been changed to 5001 and the source port for Workstation B has been changed to 5002.
If you were to look at the internal port mapping table that is maintained by the Netopia R5000 Series Router, it
would look similar to the following:
With this information the Netopia R5000 Series Router can determine the appropriate routing for an IP
response from the Internet. In this case, when the WWW server responds with a destination port of 5001, the
Netopia R5000 Series Router can see that this packet's destination on the local LAN interface is actually
Workstation A at IP address 192.168.5.2. Likewise, with the response for port 5002, the Netopia R5000
Series Router can see that this packet's destination on the local LAN interface is actually Workstation B at IP
address 192.168.5.3.
Note that this "automatic" port remapping and IP address substitution only works in one direction – for IP
packets that originated on the LAN interface destined to the WAN interface and the Internet. In order for port
remapping and IP address substitution to work in the other direction – that is, hosts on the Internet that want to
originate an IP packet destined to a host on the Netopia R5000 Series Router's LAN interface – a manual
redirection of TCP or UDP ports as well as destination IP addresses within the Netopia R5000 Series Router is
required. This manual port remapping and IP address substitution is accomplished by setting up exported
services.
Exported services are essentially user-defined pointers for a particular type of incoming TCP or UDP service
from the WAN interface to a host on the local LAN interface. This is necessary since the Netopia R5000 Series
Router and thus the attached local LAN has only one IP presence on the WAN interface and Internet. Exported
services allows the user to redirect one type of service – for example Port 21 (FTP) – to a single host on the
local LAN interface. This will then allow the Netopia R5000 Series Router to redirect any packets coming in
from the Internet with the defined destination TCP or UDP port of port 21 (FTP) to be redirected to a host on the
local LAN interface.
For example, suppose the WWW server on the Internet with the IP address of 163.176.4.32 wants to access
Workstation B on the Netopia R5000 Series Router's local LAN interface which is operating as an FTP server.
The IP address for Workstation B is 192.168.5.3, which is not a valid IP address, and thus the WWW server on
the Internet cannot use this IP address to access Workstation B.
Source LAN IP Source LAN Port
192.168.5.2 TCP 400
192.168.5.3 TCP 400
Understanding Netopia NAT Behavior C-5
Remapped LAN Port
TCP 5001
TCP 5002