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Understanding Netopia NAT Behavior C-1
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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.
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Below is a diagram of the network referenced in this appendix.
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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.
ISP Network Customer Site
Internet
Router
Netopia
LAN
Workstation B
IP: 192.168.5.3
Mask: 255.255.255.240
MAC: 00-05-02-00-1e-03
IP: 200.1.1.1
Mask: 255.255.255.0
Workstation A
IP: 192.168.5.2
Mask: 255.255.255.240
MAC: 00-05-02-04-12-4f
Netopia Router
WAN IP: 200.1.1.40
Mask: 255.255.255.0
MAC: 00-00-c5-60-21-0a
Netopia Router
LAN IP: 192.168.5.1
Mask: 255.255.255.240
MAC: 00-00-c5-60-21-0a
WWW Server
IP: 163.176.4.32
Mask: 255.255.255.0
MAC: 00-05-02-0c-1b-41