Every machine on the Internet has a
unique identifying number, called an IP Address. A typical IP address
looks like this:
216.27.61.137
To make it easier for us humans to
remember, IP addresses are normally expressed in decimal format as a
"dotted decimal number" like the one above. But
computers communicate in binary form. Look at the same IP address in
binary:
11011000.00011011.00111101.10001001
The four numbers in an IP address are
called octets, because they each have eight positions when viewed
in binary form. If you add all the positions together, you get 32, which
is why IP addresses are considered 32-bit numbers. Since each of the
eight positions can have two different states (1 or 0) the total number
of possible combinations per octet is 28 or 256. So each
octet can contain any value between 0 and 255. Combine the four octets
and you get 232 or a possible 4,294,967,296 unique values!
Out of the almost 4.3 billion possible
combinations, certain values are restricted from use as typical IP
addresses. For example, the IP address 0.0.0.0 is reserved for the
default network and the address 255.255.255.255 is used for broadcasts.
The octets serve a purpose other than
simply separating the numbers. They are used to create classes of
IP addresses that can be assigned to a particular business, government
or other entity based on size and need. The octets are split into two
sections: Net and Host. The Net section always contains
the first octet. It is used to identify the network that a computer
belongs to. Host (sometimes referred to as Node) identifies the
actual computer on the network. The Host section always contains the
last octet. There are five IP classes plus certain special addresses:
Default Network - The IP
address of 0.0.0.0 is used for the default network.
Class A - This class is for
very large networks, such as a major international company might
have. IP addresses with a first octet from 1 to 126 are part of this
class. The other three octets are used to identify each host. This
means that there are 126 Class A networks each with 16,777,214 (224
-2) possible hosts for a total of 2,147,483,648 (231)
unique IP addresses. Class A networks account for half of the total
available IP addresses. In Class A networks, the high order bit
value (the very first binary number) in the first octet is always 0.
Net
Host or Node
115.
24.53.107
Loopback - The IP address 127.0.0.0
is used as the loopback address. This means that it is used by the
host computer to send a message back to itself. It is commonly used
for troubleshooting and network testing.
Class B - Class B is used
for medium-sized networks. A good example is a large college campus.
IP addresses with a first octet from 128 to 191 are part of this
class. Class B addresses also include the second octet as part of
the Net identifier. The other two octets are used to identify each
host. This means that there are 16,384 (214) Class B
networks each with 65,534 (216 -2) possible hosts for a
total of 1,073,741,824 (230) unique IP addresses. Class B
networks make up a quarter of the total available IP addresses.
Class B networks have a first bit value of 1 and a second bit value
of 0 in the first octet.
Net
Host or Node
145.24.
53.107
Class C - Class C addresses
are commonly used for small to mid-size businesses. IP addresses
with a first octet from 192 to 223 are part of this class. Class C
addresses also include the second and third octets as part of the
Net identifier. The last octet is used to identify each host. This
means that there are 2,097,152 (221) Class C networks
each with 254 (28 -2) possible hosts for a total of
536,870,912 (229) unique IP addresses. Class C networks
make up an eighth of the total available IP addresses. Class C
networks have a first bit value of 1, second bit value of 1 and a
third bit value of 0 in the first octet.
Net
Host or Node
195.24.53.
107
Class D - Used for
multicasts, Class D is slightly different from the first three
classes. It has a first bit value of 1, second bit value of 1, third
bit value of 1 and fourth bit value of 0. The other 28 bits are used
to identify the group of computers the multicast message is intended
for. Class D accounts for 1/16th (268,435,456 or 228)
of the available IP addresses.
Net
Host or Node
224.
24.53.107
Class E - Class E is used
for experimental purposes only. Like Class D, it is different from
the first three classes. It has a first bit value of 1, second bit
value of 1, third bit value of 1 and fourth bit value of 1. The
other 28 bits are used to identify the group of computers the
multicast message is intended for. Class E accounts for 1/16th
(268,435,456 or 228) of the available IP addresses.
Net
Host or Node
232.
24.53.107
Broadcast - Messages that
are intended for all computers on a network are sent as broadcasts.
These messages always use the IP address 255.255.255.255.
