Netopia R910 - Manual (Page 192)

C-192 User’s Reference Guide
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 R910 will look at the destination por t and will find that it is
destined for por t 21 (FTP). The Netopia R910 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 R910’s local LAN inter face.
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 for handling 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 R910 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 Histor y.
■ When using NAT it is most likely that the Netopia R910 will be receiving an IP address from a “pool” of
dynamic IP addresses at the ISP. This means that the Netopia R910's IP presence on the Internet will
change with each connection. This can potentially cause problems with devices on the Internet attempting
to access ser vices like WWW and FTP ser vers or AURP par tners on the Netopia R910’s local LAN
inter face. In this case, if a dynamic IP address is assigned to the WAN inter face of the Netopia R910 each
time, the administrator of the Netopia R910 will have to notify clients who want to access ser vices on the
Netopia R910’s LAN inter face of the new IP address after each connection.
■ With NAT enabled, there cannot be two or more of the same types of ser vice accessible from the Internet
on the LAN inter face of the Netopia R910. For example, there cannot be multiple FTP ser vers (Por t 23) on
the Netopia R910's LAN inter face that can be accessible by workstations on the Internet. This is because
there is no way within the Netopia R910 and IP to distinguish between multiple ser vers using the same
por t, in this case por t 23.
■ Fictional IP addresses may be assigned on the Netopia R910’s LAN inter face. It is strongly recommended
that for the Netopia R910’s LAN inter face, 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 R910’s LAN inter face has an IP address of
100.1.1.2 while the Netopia R910’s LAN inter face is 100.1.1.2 and the local host wants to access a host
on the Internet with the address of 100.1.1.8, the Netopia R910 has no way of knowing that the 200.1.1.8
address is actually on the Internet and not on its local LAN inter face, since the local LAN inter face is
assigned the IP address range of 200.1.1.1 to 200.1.1.14.