Netopia R2121 manual Exported services, Understanding Netopia NAT Behavior D-5

Understanding Netopia NAT Behavior D-5

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

With this information the Netopia R2121 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 R2121 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 R2121 can see that this packet's destination on the local LAN interface is actually Workstation B at IP address 192.168.5.3.

Exported services

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 wishing to originate an IP packet destined to a host on the Netopia R2121s LAN interface, a manual redirection of TCP or UDP ports as well as destination IP addresses within the Netopia R2121 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 R2121 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 R2121 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 R2121s 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 can not use this IP address to access Workstation B.

The WWW Server on the Internet would then have to use the single valid IP address that was acquired on the Netopia R2121's WAN interface to access any host on the Netopia R2121's local LAN interface, since this is the only valid address for the Internet. But if the WWW Server on the Internet opens a connection to

200.1.1.40via Port 21 (FTP) and no Exported Services are defined on the Netopia R2121, the Netopia R2121 will discard the incoming packet since the Netopia R2121 itself does not perform the requested service.

Thus you can see why Exported Services are necessary. In the example above, an Exported Service needs to be defined within the Netopia R2121 redirecting any incoming IP traffic with a destination port of 21 to the host on the local LAN interface with the IP address of 192.168.5.3.

If the WWW Server on the Internet then tries to open a connection to the IP address of 200.1.1.40 with the appropriate Exported Service defined, the Netopia R2121 will look at the destination port and will find that it is destined for Port 21 (FTP). The Netopia R2121 then looks at the internal user-defined Exported Services table and finds that any incoming IP traffic from the WAN port with a destination of Port 21 (FTP) should be redirected to the IP address of 192.168.5.3 on the local LAN interface, which in this case is Workstation B.

Once the appropriate Exported Services are defined, there can be seamless communication between a host on the Internet and a host on the Netopia R2121’s local LAN interface.