A modular network design enables the network to expand by adding more modules, such as switches and routers, without a complete overhaul of the existing infrastructure. This provides a scalable solution for growing networks. A single flat network would be less scalable and could introduce broadcast issues as the number of hosts increases. Increased QoS would improve the quality of service for current conditions but would not address scalability of the network infrastructure. On the other hand, setting higher bandwidth limits on existing links might provide temporary relief but isn't a scalable solution for new devices and services.