B-4 Reference Guidethe subnet numbers. Since the InterNIC assigns the networknumber proper, it should not change, so the subnet numbers mustbe created out of bits that would otherwise be part of the hostnumbers.Subnet masksTo create subnets, the network manager must define a subnetmask, a 32-bit number that indicates which bits in an IP address areused for network and subnetwork addresses, and which are usedfor host addresses. One subnet mask should apply to all IPnetworks that are physically connected together and share a singleassigned network number. Subnet masks are often written indecimal notation, like IP addresses, but they are most easilyunderstood in binary notation. When a subnet mask is written inbinary notation, each numeral 1 indicates that the corresponding bitin the IP address is part of the network or subnet address. Each 0indicates that the corresponding bit is part of the host address. Thefollowing table shows the proper subnet masks to use for eachclass of network, when no subnets are required.To know whether subnets are being used or not, you must knowwhat subnet mask is being used—you cannot determine thisinformation simply from an IP address. Subnet mask information isconfigured as part of the process of setting up IP routers andgateways such as the Netopia Router.Class Subnet mask for a network with no subnetsA Binary: 11111111.00000000.00000000.00000000Decimal: 255.0.0.0B Binary: 11111111.11111111.00000000.00000000Decimal: 255.255.0.0C Binary: 11111111.11111111.11111111.00000000Decimal: 255.255.255.0
