And an interviewer will typically ask this general-purpose question early in the interview to start the conversation, learn more about your background, and collect some information they can use for future questions. This provides you the opportunity to not only discuss your motivations for becoming a network administrator but also to guide the conversation towards an area you are comfortable discussing.
Example #1
"As a child, I was always interested in technology. I was fascinated by how different devices could communicate with each other and share information. My interest continued throughout my high school and my college years. I knew very early that I wanted to work in this industry, but I was unsure of exactly which role. When I was exposed to network technology, I knew I had discovered the area in which I wanted to work. I focused on networking and took classes related to this technology. I also volunteered while still in school to keep our church's network up and running. This interest, education, and experience all led me to become a network administrator."