Computer network is one of the fundamental courses for college students majoring in CS, CSE and EECS. The objective of this course is to explain the basic principles and architecture of network based on TCP/IP. However, many students find the course quite abstract and difficult to understand. Inspired by the idea of "learning by doing", we propose a learning approach by asking the students to design and to implement their own protocol analyzer during the course. This task not only synthesizes the knowledge of all the important protocols ranging from data link layer to application layer, but also bridges the gap between theory and practical aspect. Promising feedbacks from students demonstrate that this method is very helpful for student to study computer network.