PROBLEM IN SUBNETs CLASSFULL AND CLASSLESS!!!!!!!

Question

SUBNET PROBLEM
In classfull we can use 10.0.0.0/8 as class A so we can start subnettting from /8/9...upto/32 so in short we have to start from /8 ..upto /8 all r NETWORK....
But in classless 10.0.0.0 we can use from /1 right or /25 or any mask
Questions::
1-why no need to start from /8 on class A
/16 on class B etc ...why ??
2-why we can use NETWORK bits upto /9 /10/15 etc ....????
3-how to know I have been ask on 10.0.0.0/8 from classA(classfull) or classless so how to calculate???
I have good picture example attached with this message...
In that I used 172.16.1.1/16 so in classfull 172.16.0.0/16 start host from /17 right??? so UPTO /16 ALL ARE NETWORK AFTER THAT SUBNET AND HOSTlike in my picture
but in classless picture calculator start from /20 as well and UPTO /20 ALL ARE NETWORK AND AFTER THAT HOST AND SUBNET??why???

Answer 1

Hi Anand,
IPv4 Ip address is 32 bit Binary address and which contains four octets.
so each octet is represented by eight bits
8 bit
7 Bit
6 Bit
5 Bit
4 Bit
3 Bit
2 Bit
1 Bit
128
64
32
16
8
4
2
1
Class A
0-127
0
1
1
1
1
1
1
1
Class B
128-191
1
0
1
1
1
1
1
1
Class C
192-224
1
1
0
1
1
1
1
1
In Classfull,Class A network i.e 8 bit value 128 will never become one.In any other bit it may be 0 or one.at most it can have a value of 127.thats how classfull network defined.
Class A Network 10.0.0.0 can be represented has
10= (128*0)+(64*0)+(32*0)+(16*0)+(8*1)+(4*0)+(2*1)+(1*0)
   =8+2=10
0=(128*0)+(64*0)+(32*0)+(16*0)+(8*0)+(4*0)+(2*0)+(1*0)
  =0
0=(128*0)+(64*0)+(32*0)+(16*0)+(8*0)+(4*0)+(2*0)+(1*0)
=0
0=(128*0)+(64*0)+(32*0)+(16*0)+(8*0)+(4*0)+(2*0)+(1*0)
  =0
For class A Subnet mask
255= (128*1)+(64*1)+(32*1)+(16*1)+(8*1)+(4*1)+(2*1)+(1*1)
   =128+64+32+16+8+4+2+1
   =255
0=(128*0)+(64*0)+(32*0)+(16*0)+(8*0)+(4*0)+(2*0)+(1*0)
  =0
0=(128*0)+(64*0)+(32*0)+(16*0)+(8*0)+(4*0)+(2*0)+(1*0)
=0
0=(128*0)+(64*0)+(32*0)+(16*0)+(8*0)+(4*0)+(2*0)+(1*0)
  =0
For class A-It will start from 0-126,subnet will be 255.0.0.0 i.e /8
For Class B it will start from 128-192,subnet will 255.255.0.0.i.e /16
For Class C,it will start from 192-224,subnet will be 255.255.255.0 i.e /24
These are classfull networks.
In Classless network,as per our requirement changes will be done to subnet mask
In your Classfulll screen diagram,you have assigined 20 bits for network and 12 bit for host and for 16 subnets.i.e /20 network
In your classless,you have assigned 24 bit for network and 8 bit for host i.e /24 network
Regards,
Guru