It is called UDP hole punching because it punches a hole in the firewall of the network which allows a packet from an outside system to successfully reach the desired client on a network using NAT. UDP hole punching is one of the most common techniques used to establish UDP connections with systems behind NAT. Two common Peer-to-Peer applications are Skype and Spotify. Also, most of the networks with NAT may drop incoming packets simply because it cannot figure out which client on the NAT the packet is directed to, or it may recognize it as an unauthorized packet, etc.

This is because at the time of initiating a connection in a Peer-to-Peer network, it is not possible to determine which packet coming from the peer is intended for which host on the network – because they have one global IP address. This also assures enhanced security but at the same time adds complications, especially while connecting to P2P (Peer-to-Peer) networks.

This allows the systems on the same network to have a single global IP address. A lot of networks use NAT (Network Address Translation) these days.

Wireshark ip skype