Cisco Systems SPA9000 manual Network Address Translation NAT, NAT Overview

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

Private IP address	External IP address
192.168.1.1	assigned by ISP

192.168.1.101 192.168.1.102

NAT Device

ISP

DHCP server

Internet

SPA9000

SIP Proxy		ITSP
	Session Border
192.168.1.100
	Controller

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>.

	Linksys SPA9000 Administrator Guide
Document Version 3.01		1-9