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UDP hole punching is a technique used in computer networking to establish a connection between two devices behind Network Address Translation (NAT) routers or firewalls. NAT devices typically prevent incoming connections initiated from outside the local network, but UDP hole punching can allow two devices to establish a direct communication channel even if they are both behind NAT.

Here's how UDP hole punching generally works:

1. **Initial Contact**: Each device behind NAT sends a UDP packet to a publicly accessible server known to both devices. This packet contains the device's local endpoint (IP address and port).

2. **Server Relay**: The server relays the public endpoint information of each device to the other device.

3. **Traversal Attempts**: Each device sends UDP packets to the other's public endpoint. These packets typically get blocked by their respective NATs, but the NAT devices create temporary mappings in their translation tables.

4. **Timing and Persistence**: Both devices continue to send UDP packets at regular intervals, which helps to keep the temporary mappings in the NAT devices' translation tables active.

5. **Establishing Connection**: Eventually, the NAT devices create mappings that allow the UDP packets to traverse through them, effectively establishing a direct communication channel between the two devices. Once the direct channel is established, communication can occur without needing the server as an intermediary.

UDP hole punching can be used in various applications such as peer-to-peer file sharing, online gaming, and VoIP communications. However, it's worth noting that its effectiveness can depend on the specific NAT implementations and network configurations involved. Additionally, UDP hole punching may not work in all scenarios, particularly when dealing with highly restrictive NAT configurations or symmetric NATs.