Understanding IP Addressing B-3Subnet masksTo create subnets, the network manager must define a subnet mask, a 32-bit number that indicates which bitsin an IP address are used for network and subnetwork addresses and which are used for host addresses. Onesubnet mask should apply to all IP networks that are physically connected together and share a single assignednetwork number. Subnet masks are often written in decimal notation like IP addresses, but they are most easilyunderstood in binary notation. When a subnet mask is written in binary notation, each numeral 1 indicates thatthe corresponding bit in the IP address is part of the network or subnet address. Each 0 indicates that thecorresponding bit is part of the host address. The following table shows the proper subnet masks to use foreach class of network when no subnets are required.To know whether subnets are being used or not, you must know what subnet mask is being used—you cannotdetermine this information simply from an IP address. Subnet mask information is configured as part of theprocess of setting up IP routers and gateways such as the Netopia R9100.Note: If you receive a routed account from an ISP, there must be a mask associated with your network IPaddress. By using the IP address with the mask you can discover exactly how many IP host addresses youactually have.To configure subnets properly, you must also be able to convert between binary notation and decimal notation.Example: Using subnets on a Class C IP internetWhen setting up IP routing with a Class A Address, or even with multiple Class C Addresses, subnetting is fairlystraightforward. Subnetting a single Class C address between two networks, however, is more complex. Thissection describes the general procedures for subnetting a single Class C network between two Netopia routersso that each can have Internet access.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
