NatTypeTester is a cross-platform NAT type testing tool that implements STUN protocols (RFC 3489 and RFC 5780) to determine network characteristics. The application provides desktop (Avalonia), Android, and command-line interfaces for testing NAT type, mapping behavior, filtering behavior, and public endpoint detection. It follows a clean layered architecture with MVVM pattern, dependency injection, and separation of concerns. The core STUN protocol implementation is packaged as a reusable NuGet library (Stun.Net) that supports UDP, TCP, TLS, and DTLS transports with SOCKS5 proxy capabilities.