Cisco SPA962-NA Administration Guide - Page 26

Technology Background Chapter 1 Introducing Linksys 900 Series IP Phones This is the preferred option because it eliminates the need for managing NAT on the 900 Series phone. If this is not available, you need to discuss with the ITSP how to use the NAT Support Parameters provided by the 900 Series phone, such as and . 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. NAT Types The ways that NAT is implemented can be 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. With symmetric NAT, all requests from the same internal IP address and port to a specific destination IP address and port are mapped to a unique external source IP address and port. If the same internal host sends a packet with the same source address and port to a different destination, a different mapping is used. Only an external host that receives a packet can send a UDP packet back to the internal host. Simple Traversal of UDP Through NAT Simple Traversal of UDP through NATs (STUN) is a protocol defined by RFC 3489, that allows a client behind a NAT device to find out its public address, the type of NAT it is behind, and the port associated on the Internet connection with a particular local port. This information is used to set up UDP communication between two hosts that are both behind NAT routers. Open source STUN software can be obtained at the following website: http://www.voip-info.org/wiki-Open+Source+VOIP+Software STUN does not work with a symmetric NAT router. To determine the type of NAT your router uses, complete the following steps: Step 1 Enable debugging on the 900 Series phone: 1. Make sure you do not have firewall running on your PC that could block the syslog port (by default this is 514). 1-12 Linksys 900 Series IP Phone Administrator Guide Document Version 3.0