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C-2 User’s Reference Guide
When the Netopia R5000 Series Router establishes a connection over its WAN interface with another router it
uses the Point-to-Point Protocol (PPP). Within PPP there is a Network Control Protocol (NCP) called Internet
Protocol Control Protocol (IPCP), which handles the negotiation of IP addresses between the two routers, in this
case the Netopia R5000 Series Router at the customer site above and the router at the Internet service
provider (ISP).
If the Netopia R5000 Series Router calls the router at the ISP with NAT disabled, the Netopia negotiates its LAN
interface address (as specified in IP Setup within the Netopia R5000 Series Router's console) with the router at
the ISP through IPCP and then sets up routing. From the diagram on the previous page you can see that the
address for the Netopia R5000 Series Router is 192.168.5.1 and the address of the router at the ISP is
200.1.1.1. Assuming that the addresses negotiated by the routers are valid and unique for the Internet, the
Netopia R5000 Series Router and the hosts on its LAN would be able to access the Internet.
If the Netopia R5000 Series Router calls the router at the ISP with NAT enabled, instead of negotiating the LAN
interface address, the Netopia R5000 Series Router suggests the address 0.0.0.0 through IPCP. When the
router at the ISP sees this all-zeros IPCP request, the router can either pull a free dynamic IP address from its
pool and assign it to the Netopia R5000 Series Router’s WAN interface or, if configured to do so, it can match
the Netopia R5000 Series Router's incoming connection profile and assign a preconfigured static IP address to
the Netopia R5000 Series Router's WAN interface.
From the diagram, you can see that the IP address assigned to the Netopia R5000 Series Router's WAN
interface is 200.1.1.40, while the IP address assigned to the LAN interface remains the same. The LAN
interface address 192.168.5.1 is thus hidden from the ISP and the Internet, and the Netopia R5000 Series
Router only has a single valid IP presence on the Internet. The LAN interface IP address for the Netopia R5000
Series Router can be any IP address; however, it is recommended that you use the IANA-specified 192.168.X.X
Class C address range, which is used for networks not attached to the Internet. This address range is
described in RFC 1597.
The dynamic IP address acquisition on the WAN interface of the Netopia R5000 Series Router is one of several
features of NAT. Another is the mapping of locally assigned IP addresses to the single globally unique IP
address acquired by the Netopia R5000 Series Router on its WAN interface. NAT employs several things to
accomplish this seamlessly. You must look at the formatting of an IP packet before IP address remapping can
be explained.
Every IP packet that is transmitted across the Netopia R5000 Series Router’s LAN interface or across the WAN
interface to the Internet contains several bits of information that indicate to any device where the packet is
going and where it came from. In particular, you have the source and destination port and source and
destination IP addresses.
A port is used within IP to define a particular type of service and could be either a Transmission Control
Protocol (TCP) port or User Datagram Protocol (UDP) port. Both TCP and UDP are protocols that use IP as the
underlying transport mechanism. The major difference between TCP and UDP is that TCP is a reliable delivery
service, whereas UDP is a “best-effort” delivery service. A list of well-known TCP or UDP ports and services can
be found in RFC 1700.
If Workstation A wants to communicate with a World Wide Web (WWW) server on the Internet and the Netopia
R5000 Series Router does not have NAT enabled, Workstation A forms an IP packet with the source IP address
of 192.168.5.2 and destination IP address of 163.176.4.32. The source port could be 400 while the
destination port would be 80 (WWW server). The Netopia R5000 Series Router then looks at this IP packet,
determines the best routing method and sends that packet on its way across the WAN interface to the WWW
server on the Internet.