The rapid expansion of cloud computing, Internet of Things (IoT), 5G communication, and distributed enterprise systems has significantly increased the complexity and scale of modern computer networks. Traditional static network architectures struggle to meet the growing demands for scalability, flexibility, performance, and security. This paper presents a comprehensive study on scalable and secure network infrastructure design using modern protocol architectures. It emphasizes the integration of layered communication models, Software-Defined Networking (SDN), Network Function Virtualization (NFV), secure routing mechanisms, and encryption-based communication frameworks. The study discusses how contemporary protocol stacks such as TCP/IP, IPv6, TLS, and QUIC contribute to improved data transmission efficiency and enhanced security. It further examines centralized control paradigms introduced by SDN controllers to optimize traffic engineering and enforce dynamic security policies. Core infrastructure components including routers, switches, firewalls, and intrusion detection systems are analyzed within scalable hierarchical and cloud-integrated architectures. Security mechanisms such as authentication, encryption, access control, zero-trust models, and anomaly detection are explored to address evolving cyber threats. The proposed infrastructure model demonstrates improved throughput, reduced latency, efficient resource utilization, and stronger resistance against attacks compared to conventional network designs. The findings highlight that adopting modern protocol architectures combined with intelligent management frameworks enables organizations to design future-ready networks capable of supporting large-scale deployments while maintaining robust security and operational efficiency.