Netopia R2121 - Manual (Page 243)

Understanding Netopia NAT Behavior D-1
A A A Ap p p pp p p pee e en n n nd d d diiiixxxx D D D D
U U U Un n n nd d d dee e errrrssssttttaa a an n n nd d d diiiin n n ng g g g N N N Nee e etttto o o op p p piiiiaa a a N N N NA A A ATT T T B B B Bee e eh h h haa a avvv viiiio o o orrrr
This appendix describes how Network Address Translation works within the Netopia R2121. The Netopia R2121
implements a power ful feature called Network Address Translation (NAT) as specified in RFC 1631. NAT is used
for IP address conser vation 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 R2121 and provides examples for setup
and use.
Network Configuration
Below is a diagram of the network referenced in this appendix.
Background
NAT is a mechanism employed within the Netopia R2121 to acquire a statically or dynamically assigned IP
address on its WAN inter face and proxy against locally assigned IP addresses on its LAN inter face. The Netopia
R2121 uses a one-to-many IP address mapping scheme, that is against a single IP address the Netopia R2121
acquires on its WAN inter face the Netopia R2121 can proxy 14, 30, or an unlimited number of IP hosts on the
LAN inter face.
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