Netopia R2121 User Reference Manual page 244

D-2 User's Reference Guide
When the Netopia R2121 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
R2121 at the customer site above and the Router at the Internet Service Provider (ISP).
If the Netopia R2121 calls the Router at the ISP with NAT disabled, the Netopia negotiates its LAN interface
address (as specified in IP Setup within the Netopia R2121's console) with the Router at the ISP through IPCP
and then sets up routing. From the previous diagram you can see that the address for the Netopia R2121 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 R2121 and the hosts on its LAN would be able
to access the Internet.
If the Netopia R2121 calls the Router at the ISP with NAT enabled, instead of negotiating the LAN interface
address the Netopia R2121 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 R2121's WAN interface or, if configured to do so, match the Netopia R2121's incoming connection
profile and assign a pre-configured static IP address to the Netopia R2121's WAN interface.
From the previous diagram, you can see that the IP address assigned to the Netopia R2121'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 R2121 only has a single valid IP
presence on the Internet. The LAN interface IP address for the Netopia R2121 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 R2121 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 R2121 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 R2121'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
R2121 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 R2121 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.
With NAT enabled the Netopia R2121 does something different. For example, suppose that Workstation A again
wants to communicate with the WWW Server on the Internet. 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 and source port could be 400 while
the destination port would be 80 (WWW server).