Class A Subnetting
All IP addresses were divided into classes when
they were first devised. The idea was that Class A addresses
were allocated to large companies. Class B to medium sized
companies and Class C to any other company who needed an IP address.
Class D is for multicasting such as streaming media and Class E is
reserved for experimental use.
You can recognise a Class A address because the
first octet is a number from 1 to 126 inclusive. So any of the
below IP addresses are class A.
10.1.2.4
120.2.3.5
125.1.2.3
Network devices recognise a Class A address as
such because the numbers are checked in binary and the first number
in the first octet always begins with the number 0. Check the
binary section for more info on that.
Class A addresses lock the first octet for network
numbers. The remaining three octets can be used for host
numbers. It works like this:
| Network |
Host |
Host |
Host |
| 20 |
23 |
2 |
4 |
In the above example 20 is the network number and
23.2.4 is a host on that network.
Please note that Class A, B and C addresses are
only called such for historical purposes now and you may find that
your company could be allocated a Class A address or even one for
your home IP address for your broadband connection.
VLSM means
that we don't have to concern ourselves with Classes of address
anymore.
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