What is the purpose of a subnet mask?

A subnet mask serves the purpose of differentiating between the network portion and the host portion of an IP address. This distinction is crucial for routing and addressing within a network. When an IP address is used alongside a subnet mask, the mask essentially tells networking devices which part of the address refers to the actual network and which part can be assigned to individual devices (hosts) on that network. This allows for effective IP address organization and enables devices within the same subnet to communicate directly with each other, while still allowing routing between different subnets.

In the context of the other options, defining a routing protocol does not involve the subnet mask; instead, it deals with how routers communicate with each other. Limiting network bandwidth is a function related to traffic management and Quality of Service (QoS), which is independent of subnetting. Additionally, managing repeaters is more about extending signal range in physical networks and does not relate to the logical addressing and segmentation that subnet masks provide.