Netopia R5200 Appendix C Understanding Netopia NAT Behavior, Network configuration, Background

Understanding Netopia NAT Behavior C-1

Appendix C

Understanding Netopia NAT Behavior

The Netopia R5000 Series Router implements a powerful feature called NAT as specified in RFC 1631. NAT is used for IP address conservation and for security purposes since there will only be a single IP “presence” on the WAN. This appendix describes the NAT functionality within the Netopia R5000 Series Router and provides examples for setup and use.

Network configuration

Below is a diagram of the network referenced in this appendix.

ISP Network

IP: 200.1.1.1

Mask: 255.255.255.0

Router

Internet

WWW Server

IP: 163.176.4.32

Mask: 255.255.255.0

MAC: 00-05-02-0c-1b-41

Customer Site
Netopia Router	Netopia Router
WAN IP: 200.1.1.40	LAN IP: 192.168.5.1
Mask: 255.255.255.0	Mask: 255.255.255.240
MAC: 00-00-c5-60-21-0a	MAC: 00-00-c5-60-21-0a
Workstation A	Netopia
IP: 192.168.5.2		LAN
Mask: 255.255.255.240
MAC: 00-05-02-04-12-4f
Workstation B
IP: 192.168.5.3
Mask: 255.255.255.240
MAC: 00-05-02-00-1e-03

Background

NAT is a mechanism employed within the Netopia R5000 Series Router to acquire a statically or dynamically assigned IP address on its WAN interface and proxy against locally assigned IP addresses on its LAN interface. The Netopia R5000 Series Router uses a one-to-many IP address mapping scheme; that is against a single IP address the Netopia R5000 Series Router acquires on its WAN interface, the Netopia R5000 Series Router can proxy 14, 30, or an unlimited number of IP hosts on the LAN interface.

In order to fully understand how NAT works, you must understand how a PPP connection is established and IP addresses are negotiated.