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Linksys SPA9000 Administrator Guide
Document Version 3.01
Chapter 1 Using the Linksys Voice System
Technology Background
Network Address Translation (NAT)
This section describes issues that arise when using the LVS on a network behind a network address
translation (NA) device. It includes the following topics:
• NAT Overview, page 1-9
• NAT Types, page 1-10
• Simple Traversal of UDP Through NAT, page 1-10
• SIP-NAT Interoperation, page 1-11
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.
A typical application of a NAT is to allow all the devices in a subscriber home network to access the
Internet through a router with a single public IP address assigned by an ISP. The IP header of the packets
sent from the private network to the public network is substituted by NAT with the public IP address and
a port assigned by the router. The receiver of the packets on the public network sees the packets as
coming from the external address instead of the private address of the device.
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.
Figure 1-5 NAT Support with Session Border Controller Provided by ITSP
The ITSP may support NAT mapping using a Session Border Controller (see Figure 1-5). This is the
preferred option because it eliminates the need for managing NAT on the SPA9000. If this is not
available, you will need to discuss with the ITSP how to use the NAT Support Parameters provided by
the SPA9000, such as <Outbound Proxy> and <STUN Server Enable>.
SIP Proxy
Internet
ITSP
NAT Device
SPA9000
192.168.1.102
ISP
DHCP
server
192.168.1.101
192.168.1.100
Session Border
Controller
Private IP address
192.168.1.1
External IP address
assigned by ISP