In the current digital age, ensuring the security and confidentiality of data during network transmission has become critically important. This paper presents the design and implementation of a secure data transmission system between two computers using a combination of the Columnar Transposition cipher and the Advanced Encryption Standard (AES), operating over the TCP/IP protocol. The system consists of two main components: a sender module that performs encryption and transmits the data, and a receiver module that decrypts and reconstructs the original content. Specifically, the message is encrypted in two layers-the first layer employs the Columnar Transposition algorithm to scramble the order of characters in the plaintext, while the second layer uses AES to encrypt the transposed message, thereby enhancing the overall security. In addition, the system integrates a data integrity verification mechanism using Hash-based Message Authentication Code (HMAC), which helps detect any tampering or transmission errors. The encrypted data is transmitted via Wi-Fi using the TCP/IP protocol, ensuring broad compatibility and ease of deployment. At the receiver's side, the system verifies data integrity, performs decryption in reverse order, and restores the original message. The system was implemented and tested on two independent computers, demonstrating stable operation, high reliability, and suitability for applications in teaching, research, or deployment in secure internal communication systems. Experimental results confirm that the proposed solution is both feasible and effective in real-world scenarios.