Netopia Router PN Series - Manual (Page 240)

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.
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.
Class Subnet mask for a network with no subnets
A Binary: 11111111.00000000.00000000.00000000
Decimal: 255.0.0.0
B Binary: 11111111.11111111.00000000.00000000
Decimal: 255.255.0.0
C Binary: 11111111.11111111.11111111.00000000
Decimal: 255.255.255.0