If the internet looks like a giant connected graph, the reality is much more fragmented. Most devices today sit inside private networks, separated from the public internet by routers performing Network Address Translation. NAT solved the IPv4 address shortage, but at the same time, it quietly broke one of the earliest design ideas of the internet: that any two hosts should be able to talk directly without extra machinery in between.
This gap is where NAT traversal comes in. It is a collection of techniques that lets two devices behind different routers find a direct path to one another. Without it, technologies like VoIP, online gaming, conferencing tools, and peer-to-peer messaging simply would not function.
This guide explores how NAT traversal works beneath the surface and why so much engineering effort exists just to make two devices exchange traffic.
Why NAT Became the Default
Originally, every device had a globally routable IPv4 address. That worked until the world realized that four billion addresses were not nearly enough. NAT arrived as a workaround: let many private devices share a single public IP by tracking connections and rewriting packets.
It worked so well that it became permanent infrastructure. Private ranges such as 10.0.0.0/8 or 192.168.0.0/16 became the norm, and home routers quietly started behaving as stateful firewalls.
The downside: inbound traffic could no longer reach private hosts. Unless your device initiated the flow, the NAT had no idea where to send an incoming packet, so it simply dropped it. For peer-to-peer systems, this was a major obstacle.
How NAT Devices Decide What to Pass and What to Drop
Every NAT keeps an internal table recording active flows. It watches packets as they leave your local network and rewrites the source IP and port before forwarding them to the public internet.
This assignment is tracked using a five-part identifier:
- Source IP
- Source Port
- Destination IP
- Destination Port
- Transport Protocol
If a returning packet matches one of these stored entries, it is considered legitimate. Anything else is uninvited and discarded. NAT traversal techniques rely on tricking the NAT into creating these entries at exactly the right moment so that what looks like unsolicited traffic is interpreted as part of an existing exchange.
Easy vs Hard NATs
Not all NATs behave the same. Some reuse the same public port for different destinations. Others allocate a completely new port every time you talk to a different server.
The first type, known as endpoint independent mapping, is friendlier to peer-to-peer applications because they can safely share their public address and port. The second type, endpoint dependent mapping (often called symmetric NAT), breaks typical hole punching techniques because the port used with a STUN server is not reused when contacting a peer.
Depending on filtering rules, some NATs allow packets from any source as long as a mapping exists, while others only permit traffic from specific IPs and ports. Traversal success rates hinge on these behaviors.
The Idea Behind Hole Punching
Hole punching is a clever trick based on timing. If two peers behind NATs send packets toward each other at about the same moment, both NATs build state entries showing an outbound attempt. When the incoming packets later arrive, they appear to match the NAT’s expectation and are allowed through.
This works best with UDP, which is naturally connectionless. The process usually involves:
- Each peer contacting a STUN server to learn its public address.
- A signaling method to exchange these discovered addresses.
- Both peers sending packets toward each other simultaneously.
If everything aligns, a path opens, and communication becomes direct.
TCP supports a similar idea in theory, called simultaneous open, but fewer NATs handle it well. UDP remains the backbone of most real-time and peer-to-peer connections for this reason.
When Both Devices Are on the Same Network
Sometimes two peers share the same router but still try to communicate using their public addresses. The router must recognize the traffic, translate it, and loop it back inside. This behavior is called hairpinning. Not all consumer routers support it, which is why frameworks like ICE test local addresses before considering public ones.
The Supporting Protocols: STUN, TURN, and ICE
STUN
STUN is the discovery tool that tells a device how it appears from the public internet. It answers questions like: What is my public IP and port? How does my NAT behave? It also helps maintain NAT state by sending periodic keepalive requests to stop the mapping from expiring.
TURN
When direct communication is impossible, TURN provides a safety net by relaying traffic. It allocates a public address on the server and forwards packets between the peers. This adds latency but guarantees connectivity, especially in restrictive corporate or mobile networks.
ICE
ICE coordinates everything. It gathers all possible connection candidates, including local addresses, STUN-derived public addresses, and relay addresses from TURN. Then it runs connectivity checks between each pair to determine the best available route. The goal is simple: try local options first, then attempt hole punching, and finally fall back to relays only if there is no other choice.
Additional Methods for Opening Ports
Some routers support explicit port-mapping protocols that let applications request inbound mappings directly.
Examples include:
- UPnP IGD
- NAT-PMP
- PCP
These work only when the user’s router is the only NAT in the path. In many modern environments, especially under ISP-managed networks, a second layer of NAT sits upstream.
The Reality of Carrier-Grade NAT
As IPv4 exhaustion continued, service providers began placing entire neighbourhoods or buildings behind a single public IP. This is called Carrier-Grade NAT. In such setups, even the most permissive home router cannot expose services because the ISP’s NAT blocks unsolicited traffic before it reaches your equipment.
Traversal still works in many cases, but success depends on how the carrier’s infrastructure handles mapping and hairpinning. It is far less predictable.
Will IPv6 Solve All This?
IPv6 removes address scarcity and gives each device a publicly reachable address again. In theory, this restores end-to-end connectivity. In practice, firewalls remain in place, and peer-to-peer systems still need a way to request inbound permission. Although traversal becomes simpler, global IPv6 adoption is uneven, and IPv4-based workarounds will continue to exist for years.
Building a Traversal System Yourself
A robust implementation usually includes:
- A UDP-based communication method
- Access to STUN servers
- A relay option for unrecoverable cases
- A signaling channel
- A strategy for keeping NAT ports alive
- Regular connectivity checks
- End-to-end encryption for data safety
Applications typically start with the slowest but guaranteed method, then upgrade to direct paths as better routes become available.
Tools That Handle Traversal for You
Several platforms incorporate these techniques under the hood. WebRTC embeds ICE into browsers, enabling peer-to-peer video, audio, and data. Mesh networking tools use similar logic to connect devices without manual configuration. Some tunneling utilities take a different approach altogether by creating a secure path around the NAT instead of through it.
Closing Thoughts
NAT traversal is a response to a problem created by necessity. The internet grew faster than IPv4 could support, and NAT became the glue holding everything together. Traversal techniques restore the illusion of a global, uniformly connected network even though the underlying structure is anything but simple.
Until IPv6 dominates, these mechanisms remain essential. Every smooth video call, multiplayer game, or peer-to-peer transfer is powered by countless background checks, timed packets, and clever negotiations, all designed to help one device reach another tucked away behind a private address.
Top comments (0)