Network Address Translation; Nat Overview - Linksys 900 Series Administrator's Manual

Chapter 1 Introducing Linksys 900 Series IP Phones
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If security is not a concern in your environment, you can consider disabling SPI, if this function
exists on your firewall.

Network Address Translation

This section describes issues that arise when using the LVS system on a network behind a
network address translation (NAT) device. It includes the following topics:
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NAT Overview

Network Address Translation (NAT) allows multiple devices to share the same public,
routable, IP address for establishing connections over the Internet. NAT is typically performed
by a router that forwards packets between the Internet and the internal, private network.
The association between a private address and port and a public address and port is called a
NAT mapping. This mapping is maintained for a short period of time, that varies from a few
seconds to several minutes. The expiration time is extended whenever the mapping is used to
send a packet from the source device.
The ITSP may support NAT mapping using a Session Border Controller (see
Figure 1-3
192.168.1.101
Document Version 3.1
SIP ports—By default, UDP port 5060 and 5061
RTP ports—16384 to 16482
NAT Overview, page 1-11
NAT Types, page 1-12
Simple Traversal of UDP Through NAT, page 1-12
SIP-NAT Interoperation, page 1-13
NAT Support with Session Border Controller Provided by ITSP
192.168.1.102
SPA9000
Private IP address
192.168.1.1
NAT Device
DHCP
server
SIP Proxy
192.168.1.100
Linksys 900 Series IP Phone Administrator Guide
Technology Background
Figure
External IP address
assigned by ISP
Internet
ISP
ITSP
Session Border
Controller
1-3).
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