The Session Traversal Utilities for NAT (STUN) protocol is a network protocol that is used to determine the presence of a network address translator (NAT) or firewall between a client and a server. STUN is used by applications that need to establish real-time communication between two hosts, such as voice over IP (VoIP) and video conferencing.
STUN works by sending a request to a STUN server, which responds with the public IP address and port of the client. This allows the client to determine its own external IP address and port, even if it is behind a NAT or firewall. STUN can also be used to determine the type of NAT or firewall that is being used, which can be helpful for troubleshooting connectivity issues.