An IP address is a 32-bit number that uniquely identifies a host (computer
or other device, such as a printer or router) on a TCP/IP network.
IP addresses are normally expressed in dotted-decimal format, with four
numbers separated by periods, such as 192.168.123.132. To understand how
subnet masks are used to distinguish between hosts, networks, and
subnetworks, examine an IP address in binary notation.
For example, the dotted-decimal IP address 192.168.123.132 is (in binary
notation) the 32 bit number 110000000101000111101110000100. This number
may be hard to make sense of, so divide it into four parts of eight binary
digits.
These eight bit sections are known as octets. The example IP address,
then, becomes 11000000.10101000.01111011.10000100. This number only makes
a little more sense, so for most uses, convert the binary address into
dotted-decimal format (192.168.123.132). The decimal numbers separated by
periods are the octets converted from binary to decimal notation.
For a TCP/IP wide area network (WAN) to work efficiently as a collection
of networks, the routers that pass packets of data between networks do not
know the exact location of a host for which a packet of information is
destined. Routers only know what network the host is a member of and use
information stored in their route table to determine how to get the packet
to the destination host's network. After the packet is delivered to the
destination's network, the packet is delivered to the appropriate host.
For this process to work, an IP address has two parts. The first part of
an IP address is used as a network address, the last part as a host
address. If you take the example 192.168.123.132 and divide it into these
two parts you get the following:
192.168.123. Network
.132 Host
-or-
192.168.123.0 - network address.
0.0.0.132 - host address.
Subnet mask
The second item, which is required for TCP/IP to work, is the subnet mask.
The subnet mask is used by the TCP/IP protocol to determine whether a host
is on the local subnet or on a remote network.
In TCP/IP, the parts of the IP address that are used as the network and
host addresses are not fixed, so the network and host addresses above
cannot be determined unless you have more information. This information is
supplied in another 32-bit number called a subnet mask. In this example,
the subnet mask is 255.255.255.0. It is not obvious what this number means
unless you know that 255 in binary notation equals 11111111; so, the
subnet mask is:
11111111.11111111.11111111.0000000
Lining up the IP address and the subnet mask together, the network and
host portions of the address can be separated:
11000000.10101000.01111011.10000100 -- IP address (192.168.123.132)
11111111.11111111.11111111.00000000 -- Subnet mask (255.255.255.0)
The first 24 bits (the number of ones in the subnet mask) are identified
as the network address, with the last 8 bits (the number of remaining
zeros in the subnet mask) identified as the host address. This gives you
the following:
11000000.10101000.01111011.00000000 -- Network address (192.168.123.0)
00000000.00000000.00000000.10000100 -- Host address (000.000.000.132)
So now you know, for this example using a 255.255.255.0 subnet mask, that
the network ID is 192.168.123.0, and the host address is 0.0.0.132. When a
packet arrives on the 192.168.123.0 subnet (from the local subnet or a
remote network), and it has a destination address of 192.168.123.132, your
computer will receive it from the network and process it.
Almost all decimal subnet masks convert to binary numbers that are all
ones on the left and all zeros on the right. Some other common subnet
masks are:
Decimal Binary
255.255.255.192 1111111.11111111.1111111.11000000
255.255.255.224 1111111.11111111.1111111.11100000
Internet RFC 1878 (available from
http://www.internic.net
(http://www.internic.net)
) describes the valid subnets and subnet masks that can be used on TCP/IP networks.
