Subnetting allows a large network block to be logically segmented into multiple smaller networks, or subnets. The use of many smaller networks allows the use of varying physical networks, such as Ethernet or Fiber Optic, which could not otherwise be combined. Additionally, the smaller subnets can improve the speed of traffic and permit easier management.Subnet masks provide the logical division required by routers to be able to address logical subnets. A default gateway IP address on a router interface allows clients to access networks outside their local subnet segment. The number of bits used in a subnet mask governs the number of subnets available.