P-2302R Series VoIP Station Gateway User's Guide

P-2302R Series User’s Guide
Chapter 7 Introduction to VoIP 94
7.3 NAT
NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a
host in a packet. For example, the source address of an outgoing packet, used within one
network is changed to a different IP address known within another network.
In the simplest form, NAT changes the source IP address of a packet received from a device to
another IP address before forwarding the packet towards the destination. When the response
comes back, NAT translates the destination address back to the device’s IP address and
forwards it to the device.
NAT routers are commonly used to translate private (or internal) IP addresses in packet
headers to public (or external) IP addresses and vice versa. A NAT router maps a private IP
address and port pair to a public IP address and port, and whenever the NAT router receives a
packet with that public IP address and port, it knows how to reroute the packet back to the
private IP address and port.
NAT may be implemented on a device that is between your Prestige and the Internet.
7.3.1 NAT Example
See the following figure. The Prestige (X) sends packets to the Internet. The Prestige’s IP
address is 10.0.0.3. This is a private or internal IP address. The NAT router maps the private
source IP address to a public source IP address (a.b.c.d). The public source IP address is also
known as the external IP address.
Note: The NAT figures in this chapter use lower-case letters (like a.b.c.d for example)
to represent public IP addresses.
Figure 30 NAT: Outgoing
When the NAT router receives packets with destination address IP address a.b.c.d, the NAT
router changes a.b.c.d back to the private IP address 10.0.0.3 and sends it to the Prestige.