The 1st class is once again Class A when looking at the 1st octet
of an IPv4 Address, in other words if we've got x.x.x.x
we're looking at the 1st octet or 1st 8 bits
the 1st octet always starts with the binary 0
so in the 1st octet or the 1st 8 bits
the 1st bit is set to 0, now if we go through all
the combinations of binary in the 1st octet that means we'll end
up with combinations from 0 in decimal up to 127 in decimal
So the range for Class A Addresses is from 0.0.0.0
up to 127.255.255.255
So for completeness let's briefly look at that
Class A in the 1st octet has binary values
where the 1st bit is set to 0
so the 1st 8 entry would be 00000000 which equates to 0 in decimal
the next binary entry is 00000001 which equates to decimal 1
going through all the combinations 2, 3, 4 and so forth
and so on will end up at 127
So in the 1st octet a Class A Address always starts with 0
for the 1st bit and the combinations are from 0 to 127 in decimal
Now as always in life there are some exceptions
So in Class A there are exception where 127 is reserved for
the loopback, I'll explain the loopback Address
in more detail in a moment, you cannot as an example configure
an IP Address of 127.0.0.1 on a PC
that is not an address that's available for you to configure
manually as a Static Address on a PC
0 is reserved for the default Network so that can't be used
either to configure an IP Address on a PC
So you couldn't give your PC an IP Address of 0.1.1.1
So the actual range for Class A Addresses
is the range 1.0.0.0 going through all the combinations
up to 126.255.255.255
So in the first octet, the values are from 1 to 126
for Class A Addresses
Notice that on my PC as an example
if I go to the local area connection properties and select
IPv4 and then I'll try and configure IP Address of 127
The PC tells me that that's not possible, IP Addresses starting
with 127 are not valid because they're reserved for Loopback
Addresses, we need to specify an IP Address in the range
1 to 223, in the same way if we try and configure an IP Address
of 0, Windows in this example tells us that that IP Address
is not valid and we need to specify a value between 1 and 223
So Class A Addresses 1 to 126 in the 1st octet
So in summary, in a Class A Address
the 1st 8 bits denote Network and the last 24 bits denote host
this is determined by the IANA
So an address like 10.1.1.1 implies that this is a Class A
Address, the 1st 8 bits is Network
and the remaining 24 bits is host
The next Class of Address is Class B
Class B Addresses start with Binary 10
Please note that's not 10 in decimal it's binary 10
Class A Addresses at the 1st bit in the 1st octet
set to 0, on Class B Addresses the 2nd bit position
in the 1st octet is set to 0
If you go through all the combinations, you'll end up with
numbers in the range 128 to 191
So Class B Addresses are therefore in the range
128 to 191 in the 1st octet
So in Class B Addresses the 1st 16 bits denote Network
the 2nd 16 bits denote the host portion
So 1st 2 octets are Network, the last 2 octets are the host
portion of the address
So for example 172.16.1.1
the 172 in the 1st octet tells us that this is a Class B Address
because it's in the range 128 to 191
So 172.16 is the Network portion
1.1 is the host portion of this Address
and we know that because this is the Class B Address