Netopia R910 User Manual

Now both IP packets have the exact same source IP address (200.1.1.40) and source por ts (400). The Netopia 
R910 is then able to distinguish between the two IP packets by changing the source TCP or UDP por ts and 
keeping this information in an internal table. As seen above, the source por t for Workstation A has been 
changed to 5001 and the source por t for Workstation B has been changed to 5002.

If you were to look at the internal por t mapping table that is maintained by the Netopia R910, it would look 
similar to the following:

With this information the Netopia R910 can determine the appropriate routing for an IP response from the 
Internet. In this case, when the WWW ser ver responds with a destination por t of 5001, the Netopia R910 can 
see that this packet's destination on the local LAN inter face is actually Workstation A at IP address 
192.168.5.2. Likewise, with the response for por t 5002, the Netopia R910 can see that this packet's 
destination on the local LAN inter face is actually Workstation B at IP address 192.168.5.3.

Note that this “automatic” por t remapping and IP address substitution only works in one direction – for IP 
packets that originated on the LAN inter face destined to the WAN inter face and the Internet. In order for por t 
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 R910s LAN inter face – a manual redirection of TCP or 
UDP por ts as well as destination IP addresses within the Netopia R910 is required. This manual por t 
remapping and IP address substitution is accomplished by setting up expor ted ser vices.

Expor ted ser vices are essentially user-defined pointers for a par ticular type of incoming TCP or UDP ser vice 
from the WAN inter face to a host on the local LAN inter face. This is necessar y since the Netopia R910 and 
thus the attached local LAN has only one IP presence on the WAN inter face and Internet. Expor ted ser vices 
allows the user to redirect one type of ser vice – for example Por t 21 (FTP) – to a single host on the local LAN 
inter face. This will then allow the Netopia R910 to redirect any packets coming in from the Internet with the 
defined destination TCP or UDP por t of por t 21 (FTP) to be redirected to a host on the local LAN inter face.

For example, suppose the WWW ser ver on the Internet with the IP address of 163.176.4.32 wants to access 
Workstation B on the Netopia R910s local LAN inter face which is operating as an FTP ser ver. The IP address for 
Workstation B is 192.168.5.3, which is not a valid IP address, and thus the WWW ser ver on the Internet cannot 
use this IP address to access Workstation B.

The WWW ser ver on the Internet would then have to use the single valid IP address that was acquired on the 
Netopia R910's WAN inter face to access any host on the Netopia R910's local LAN inter face, since this is the 
only valid address for the Internet. But if the WWW ser ver on the Internet opens a connection to 200.1.1.40 via 
por t 21 (FTP) and no expor ted ser vices are defined on the Netopia R910, the Netopia R910 will discard the 
incoming packet since the Netopia R910 itself does not per form the requested ser vice.

You can see why expor ted ser vices are necessar y. In the example above, an Expor ted Ser vice needs to be 
defined within the Netopia R910 redirecting any incoming IP traffic with a destination por t of 21 to the host on 
the local LAN inter face with the IP address of 192.168.5.3. 

                  Source LAN IP    Source LAN Port    Remapped LAN Port
                  192.168.5.2      TCP 400            TCP 5001
                  192.168.5.3      TCP 400            TCP 5002