Netopia R2121 - Manual (Page 247)

Understanding Netopia NAT Behavior D-5
With this information the Netopia R2121 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 R2121 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 R2121 can see that this packet's
destination on the local LAN inter face is actually Workstation B at IP address 192.168.5.3.
Exported services
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 wishing to
originate an IP packet destined to a host on the Netopia R2121s LAN inter face, a manual redirection of TCP or
UDP por ts as well as destination IP addresses within the Netopia R2121 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 R2121 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 R2121 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 R2121s 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 can
not 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 R2121's WAN inter face to access any host on the Netopia R2121'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 R2121, the Netopia R2121
will discard the incoming packet since the Netopia R2121 itself does not per form the requested ser vice.
Thus 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 R2121 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.
If the WWW Ser ver on the Internet then tries to open a connection to the IP address of 200.1.1.40 with the
appropriate Expor ted Ser vice defined, the Netopia R2121 will look at the destination por t and will find that it is
destined for Por t 21 (FTP). The Netopia R2121 then looks at the internal user-defined Expor ted Ser vices table
and finds that any incoming IP traffic from the WAN por t with a destination of Por t 21 (FTP) should be redirected
to the IP address of 192.168.5.3 on the local LAN inter face, which in this case is Workstation B.
Once the appropriate Expor ted Ser vices are defined, there can be seamless communication between a host on
the Internet and a host on the Netopia R2121’s local LAN inter face.
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