The L-band Digital Aeronautical Communications System (LDACS) is a cellular-based broadband, secure digital aeronautical communications system designed to enhance the safety and efficiency of air traffic management (ATM) through the facilitation of innovative ATM paradigms, such as 4D trajectory-based operations (TBO). LDACS is in its final stages of standardization by the International Civil Aviation Organization (ICAO) and the European Organisation for Civil Aviation Equipment (EUROCAE). Furthermore, it has been introduced to not only the aviation industry but also the Internet community in the form of an informational Request For Comments (RFC). Rapidly creating prototypes of an international communications standard by multiple organizations is crucial to the effective deployment of that standard. This approach enables validation and interoperability testing across various countries' prototypes. Therefore, first, we create an LDACS prototype through a software/hardware co-design strategy by Software Defined Radio (SDR) and High-Level Synthesis (HLS). This approach expedites and economically streamlines the development process. Second, we show the alignment of our prototype with the LDACS specification through preliminary and cell entry tests. Third, we demonstrate the efficacy of LDACS' Quality of Service (QoS) and security features via end-to-end IPv6 connectivity and security tests. Finally, the soundness and clarity of the LDACS specification is evidenced via interoperability tests between our prototype and a European counterpart.