Network Security is second only to the performance of the network. Some people may even argue that it is the primary element you need a focus on. As a network administrator, you should be able to discuss your skills and experience implementing the security features of a network. You may want to talk about specific technologies and provide the interviewer with examples of how you have successfully done this in your previous positions.
"When designing a network infrastructure, one of the key elements I focus on is security. If the network is not secure, performance is irrelevant because bad actors can penetrate the network, compromise information, and insert malware, which causes additional damage to the organization. I have a great deal of experience in implementing network security features. This includes creating firewalls, managing user authentication, tracing packets within the network to assure that they are legitimate, and managing both vulnerabilities in threats as they emerge."