Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series IP Phones
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internal private ones. NAT ensures security since each outgoing or incoming request must first
go through a translation process.
NAT Types
Symmetrical NAT
In symmetrical NAT, the NAT router stores the address and port where the packet was sent. Only
packets coming from this address and port are forwarded back to the private address.
Full Cone NAT
In full cone NAT, all packets from a private address (e.g., iAddr: port1) to public network will be
sent through a public address (e.g., eAddr: port2). Packets coming from the address of any
server to eAddr: port2 will be forwarded back to the private address (e.g., iAddr: port1).
Address Restricted Cone NAT
Restricted cone NAT works similar like full cone NAT. A public host (hAddr: any) can send
packets to iAddr: port1 through eAddr: port2 only if iAddr: port1 has previously sent a packet to
hAddr: any. "Any" means the port number doesn't matter.
Port Restricted Cone NAT
Port restricted cone NAT works similar like full cone NAT. A public host (hAddr: hPort) can send
packets to iAddr: port1 through eAddr: port2 only if iAddr: port1 has previously sent a packet to
hAddr: hPort.
NAT Traversal
In the VoIP environment, NAT breaks end-to-end connectivity.
NAT traversal is a general term for techniques that establish and maintain IP connections
traversing 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 the
manually configured external IP address for NAT traversal. The TURN protocol is used as part of
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