Netopia R7100-C User Reference Manual

Contents

Netopia R7100-C User Reference Manual Manual pdf 226 page image

C-6 User’s Reference GuideIf the WWW server on the Internet then tries to open a connection to the IP address of 200.1.1.40 with theappropriate Exported Service defined, the Netopia R7100 will look at the destination port and will find that it isdestined for port 21 (FTP). The Netopia R7100 then looks at the internal user-defined exported services tableand finds that any incoming IP traffic from the WAN port with a destination of port 21 (FTP) should be redirectedto 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 onthe Internet and a host on the Netopia R7100’s local LAN interface.Important notesEven 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. Themajority of router vendors use the all-zeros IPCP request to determine when a dial-in host wants to beassigned an IP address. Some vendors however attempt to negotiate and establish routing with anall-zeros IP address. The Netopia R7100 will not allow routing to be established with an all-zeros IPaddress 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 R7100 will be receiving an IP address from a “pool” ofdynamic IP addresses at the ISP. This means that the Netopia R7100's IP presence on the Internet willchange with each connection. This can potentially cause problems with devices on the Internet attemptingto access services like WWW and FTP servers or AURP partners on the Netopia R7100’s local LANinterface. In this case, if a dynamic IP address is assigned to the WAN interface of the Netopia R7100each time, the administrator of the Netopia R7100 will have to notify clients who want to access serviceson the Netopia R7100’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 Interneton the LAN interface of the Netopia R7100. For example, there cannot be multiple FTP servers (Port 23) onthe Netopia R7100's LAN interface that can be accessible by workstations on the Internet. This is becausethere is no way within the Netopia R7100 and IP to distinguish between multiple servers using the sameport, in this case port 23. Fictional IP addresses may be assigned on the Netopia R7100’s LAN interface. It is strongly recommendedthat for the Netopia R7100’s LAN interface, an IP address from the Class C address range of 192.168.X.Xbe used. This is because this range is defined by the IANA as an address space that will never be routedthrough 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 isused instead, this address space can potentially overlap an address space that is owned by a userattached to the Internet. Thus if a user on the Netopia R7100’s LAN interface has an IP address of100.1.1.2 while the Netopia R7100’s LAN interface is 100.1.1.2 and the local host wants to access ahost on the Internet with the address of 100.1.1.8, the Netopia R7100 has no way of knowing that the200.1.1.8 address is actually on the Internet and not on its local LAN interface, since the local LANinterface is assigned the IP address range of 200.1.1.1 to 200.1.1.14.