B-4 Reference Guide
the subnet numbers. Since the InterNIC assigns the network number proper, it should not change, so the subnet numbers must be created out of bits that would otherwise be part of the host numbers.
Subnet masks
To create subnets, the network manager must define a subnet mask, a 32-bit number that indicates which bits in an IP address are used for network and subnetwork addresses, and which are used for host addresses. One subnet mask should apply to all IP networks that are physically connected together and share a single assigned network number. Subnet masks are often written in decimal notation, like IP addresses, but they are most easily understood in binary notation. When a subnet mask is written in binary notation, each numeral 1 indicates that the corresponding bit in the IP address is part of the network or subnet address. Each 0 indicates that the corresponding bit is part of the host address. The following table shows the proper subnet masks to use for each class of network, when no subnets are required.
Class | Subnet mask for a network with no subnets |
ABinary: 11111111.00000000.00000000.00000000
Decimal: 255.0.0.0
BBinary: 11111111.11111111.00000000.00000000
Decimal: 255.255.0.0
CBinary: 11111111.11111111.11111111.00000000
Decimal: 255.255.255.0
To know whether subnets are being used or not, you must know what subnet mask is being used—you cannot determine this information simply from an IP address. Subnet mask information is configured as part of the process of setting up IP routers and gateways such as the Netopia Router.
