The interviewer has already asked you what you know about their company. Now they are asking you what you know about their network infrastructure. Many job candidates will not have done their homework and will need to ask the interviewer some questions to qualify this. However, as a well-qualified candidate, you should have done your research and already have a good understanding of their network topology. You can find this information on their website, in public documents they have released, from other job search sites such as Glassdoor, and by conducting informational interviews with some of their employees. You can also ask qualifying questions of the interviewer once you begin to describe the topology.