Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series IP Phones90internal private ones. NAT ensures security since each outgoing or incoming request must firstgo through a translation process.NAT TypesSymmetrical NATIn symmetrical NAT, the NAT router stores the address and port where the packet was sent. Onlypackets coming from this address and port are forwarded back to the private address.Full Cone NATIn full cone NAT, all packets from a private address (e.g., iAddr: port1) to public network will besent through a public address (e.g., eAddr: port2). Packets coming from the address of anyserver to eAddr: port2 will be forwarded back to the private address (e.g., iAddr: port1).Address Restricted Cone NATRestricted cone NAT works similar like full cone NAT. A public host (hAddr: any) can sendpackets to iAddr: port1 through eAddr: port2 only if iAddr: port1 has previously sent a packet tohAddr: any. "Any" means the port number doesn't matter.Port Restricted Cone NATPort restricted cone NAT works similar like full cone NAT. A public host (hAddr: hPort) can sendpackets to iAddr: port1 through eAddr: port2 only if iAddr: port1 has previously sent a packet tohAddr: hPort.NAT TraversalIn the VoIP environment, NAT breaks end-to-end connectivity.NAT traversal is a general term for techniques that establish and maintain IP connectionstraversing NAT gateways, typically required for client-to-client networking applications,especially for VoIP deployments. Yealink IP phones support three NAT traversal techniques:manual NAT, STUN and ICE. If manual NAT and STUN are all enabled, the IP phone will use themanually configured external IP address for NAT traversal. The TURN protocol is used as part of
