Cisco SPA1001 Administration Guide - Page 25

Chapter 1 Introducing Linksys Analog Telephone Adapters Technology Background 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 NAT Support with Session Border Controller Provided by ITSP Private IP address 192.168.1.1 External IP address assigned by ISP Linksys ATA 192.168.1.100 NAT Device ISP DHCP server Internet NAT Types ITSP Session Border Controller The ITSP may support NAT mapping using a Session Border Controller (see Figure 1-3). This is the 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 and . The different ways that NAT is implemented is sometimes divided into the following categories: • 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. Document Version 3.1 Linksys ATA Administrator Guide 1-11