NAT (Network Address Translation)
NAT lets many devices share one public IP address by rewriting packet addresses at the router. It breaks SIP and RTP because they carry addresses inside their messages.
Published 8 November 2026 · 1 min read · By SpaGreen Creative
Short answer
NAT is the router function that maps private internal IP addresses to one or more public addresses, which breaks SIP and RTP because both protocols embed addresses inside their messages.
Almost every home and office network uses NAT. Devices inside have private addresses such as 192.168.1.20, and the router rewrites the source address of outgoing packets to its single public address, remembering the mapping so that replies can be sent back to the right device. RFC 3022 describes the traditional form.
NAT works transparently for protocols such as HTTP, where the address in the packet header is all that matters. SIP and RTP are different. A SIP client writes its own IP address and port into the Via and Contact headers and into the SDP body. Behind NAT, it writes the private address, which is meaningless to the server and to the other party. The server may send the INVITE or the audio to 192.168.1.20 on the public internet, where it goes nowhere.
The symptoms are familiar: registration succeeds but calls do not ring, or calls connect with one-way or no audio. The fixes work from both sides. The client can discover its public address with STUN and negotiate reachable paths with ICE. The server can ignore the advertised addresses and reply to where packets actually came from, which Asterisk does with nat=yes or force_rport,comedia. A VPN avoids the problem by giving every device a routable address.
Sources and further reading
- RFC 3022: Traditional IP Network Address Translator (Traditional NAT)www.rfc-editor.org
- RFC 3261: SIP: Session Initiation Protocolwww.rfc-editor.org
This article is published by SpaGreen Creative, the maker of SIPdia. Product names mentioned belong to their owners. Where it describes other software, treat the steps as general guidance and check the official documentation linked above.
