Nat Types - Linksys SPA1001 Administrator User Manual

Chapter 1 Introducing Linksys Analog Telephone Adapters
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 1-3
Linksys ATA
192.168.1.100
The ITSP may support NAT mapping using a Session Border Controller (see
preferred option because it eliminates the need for managing NAT on the Linksys ATA. If this is not
available, you will need to discuss with the ITSP how to use the NAT Support Parameters provided by
the Linksys ATA, such as <Outbound Proxy> and <STUN Server Enable>.

NAT Types

The different ways that NAT is implemented is sometimes divided into the following categories:
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Document Version 3.1
NAT Support with Session Border Controller Provided by ITSP
Private IP address
192.168.1.1
NAT Device
DHCP
server
Full cone NAT—Also known as one-to-one NAT. All requests from the same internal IP address and
port are mapped to the same external IP address and port. An external host can send a packet to the
internal host, by sending a packet to the mapped external address
Restricted cone NAT—All requests from the same internal IP address and port are mapped to the
same external IP address and port. Unlike a full cone NAT, an external host can send a packet to the
internal host only if the internal host had previously sent a packet to it.
Port restricted cone NAT/symmetric NAT—Port restricted cone NAT or symmetric NAT is like a
restricted cone NAT, but the restriction includes port numbers. Specifically, an external host can
send a packet to a particular port on the internal host only if the internal host had previously sent a
packet from that port to the external host.
External IP address
assigned by ISP
Internet
ISP
ITSP
Session Border
Controller
Technology Background
Figure 1-3). This is the
Linksys ATA Administrator Guide
1-11