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Networking Concepts A-7
Network Addressing
Netmasks
A netmask is a four-octet number that identifies either a supernetwork (supernet) or a
subnetwork (subnet). A netmask that designates a subnet is called a subnet mask.
Using Subnet Masks to Create IP Subnets
Subnet masks are used to divide networks into smaller, more manageable groups of
hosts known as subnets. Subnetting is a scheme for imposing a hierarchy on hosts on a
single physical network. The usual practice is to use the first few bits in the host portion
of the network address for a subnet field. RFC 950,
Internet Standard Subnetting Procedure
,
describes subnetting.
A subnet mask identifies the subnet field of a network address. This mask is a 32-bit
number written in dotted decimal notation with all 1s (ones) in the network and subnet
portions of the address, and all 0s (zeros) in the host portion. This scheme allows for the
identification of the host portion of any address on the network.
Table A-3 shows the subnet masks you can use to divide a class C network into subnets.
Table A-3
Subnet Masks for a Class C Network
Length
(Subnet
Bits)
Number of
Subnets
Number of
Hosts per
Subnet
Hexadecimal
Subnet Mask
Dotted Decimal
Subnet Mask
24 1 254 0xffffff00 255.255.255.0
25 2 126 0xffffff80 255.255.255.128
26 4 62 0xffffffc0 255.255.255.192
27 8 30 0xffffffe0 255.255.255.224
28 16 14 0xfffffff0 255.255.255.240
29 32 6 0xfffffff8 255.255.255.248
30 64 2 0xfffffffc 255.255.255.252
32 256 1 0xffffffff 255.255.255.255