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D-6 User’s Reference Guide
Important notes
Even with the advantages of NAT, there are several things you should note carefully:
■ There is no formally agreed upon method among router vendors to handle an all-zeros IPCP request. The
majority of router vendors use the all-zeros IPCP request to determine when a dial-in host wants to be
assigned an IP address. Some vendors however attempt to negotiate and establish routing with an
all-zeros IP address. The Netopia R2121 will not allow routing to be established with an all-zeros IP
address and the call will be dropped with an error logged in the Device Event History.
■ When using NAT it is most likely that the Netopia R2121 will be receiving an IP address from a “pool” of
dynamic IP addresses at the ISP. This means that the Netopia R2121's IP presence on the Internet will
change with each connection. This can potentially cause problems with devices on the Internet attempting
to access services like WWW and FTP servers or AURP partners on the Netopia R2121’s local LAN
interface. In this case, if a dynamic IP address is assigned to the WAN interface of the Netopia R2121
each time, the administrator of the Netopia R2121 will have to notify clients wishing to access services on
the Netopia R2121’s LAN interface of the new IP address after each connection.
■ With NAT enabled, there cannot be two or more of the same types of service accessible from the Internet
on the LAN interface of the Netopia R2121. For example, there cannot be multiple FTP servers (Port 23) on
the Netopia R2121's LAN interface that can be accessible by workstations on the Internet. This is due to
the fact that within the Netopia R2121 and IP there is no way to distinguish between multiple servers using
the same port, in this case port 23.
■ Fictional IP addresses may be assigned on the Netopia R2121’s LAN interface. It is strongly recommended
that for the Netopia R2121’s LAN interface, an IP address from the Class C address range of 192.168.X.X
be used. This is because this range is defined by the IANA as an address space that will never be routed
through the Internet and is to be used by private Intranets not attached to the Internet.
If the address range of 192.168.X.X is not used and another range of addresses such as 100.1.1.X is
used instead, this address space can potentially overlap an address space that is owned by a user
attached to the Internet. Thus if a user on the Netopia R2121’s LAN interface has an IP address of
100.1.1.2 while the Netopia R2121’s LAN interface is 100.1.1.2 and the local host wishes to access a
host on the Internet with the address of 100.1.1.8, the Netopia R2121 has no way of knowing that the
200.1.1.8 address is actually on the Internet and not on its local LAN interface, as the local LAN interface
is assigned the IP address range of 200.1.1.1 to 200.1.1.14.
Configuration
Network Address Translation is enabled by default with the SmartStart configuration utility. You can toggle
Enable Address Translation to NO or YES in the Connection Profile screen in System Configuration under the IP
Profile Parameters section. NAT is enabled on a per-profile basis, so it is possible to have any combination of
NAT and non-NAT profiles. An example of enabling NAT is as follows: