Classful Addressing

[drago11]

I'm having trouble figuring out what some of this means so i'll break it down in parts:
With classful addressing, the address always has an 8, 16 or 24-bit network field, based on the Class A, B, and C addressing rules. (I understand this statement)

So

255.0.0.0
255.255.0.0
255.255.255

Class A,B, and C respectively
The end of the address has a host part that uniquely identifies each host inside a subnet. (I understand this, the zeros)
The bits in between the network and host part comprise the third part, namely the subnet part of the address.
(What does this mean)
I'm having trouble understanding the terminology. Can someone help me out.

[bhavesh96el]

It is very simple let me try to explain briefly but i would suggest you read "subnetting" :
Lets take example of class B ,

Ip address :- 172.30.0.1 255.255.0.0

that means network address : 172.30.0.0 and Broadcast address for this network : 172.30.255.255
Total number of possible host = 172.30.0.1 to 172.30.255.254 ( i.e 2^16 - 2 = 65534 )

Now suppose i don't need that many host so i can divide this one network into small subnets by borrowing bits from host and giving it to network.
lets suppose i borrow 8 bits from host and giving it to network
in this case

ip address :172.30.0.1 255.255.255.0

that means network address 172.30.0.0 and broadcast address for this network 172.30.0.255

possible hosts :- 172.30.0.1 to 172.30.0.254 ( i.e 2^8-2 = 254 )

now lets look at total subnet created from Class B network ( 172.30.0.0 with 65534 hosts )

subnetwork 1 : 172.30.0.0 with total 254 hosts
subnetwork 2 : 172.30.1.0 with total 254 hosts
subnetwork 3: 172.30.2.0 with total 254 hosts
..
..
..
..
..
subnetwork 255: 172.30.255.0 with total 254 hosts

so by borrowing 8 bits from host and giving it to network divided 1 Class B network in 255 subnetworks.
so that borrowed 8 bits are the subnet bits that you are confused about.
I hope this clear your doubt.

[sunilabraham]

Hi Drago,

In classful addressing the bits between the network bits and host bits are the subnet bits. Subnet bits are actually taken out of host bits to make the number of subnets required in each network.

For example, class c network has 8 host bits. If the network requires 6 hosts and 32 subnets we may take 3 bits out of the 8 host bits and form 6 hosts( after removing the two reserved hosts) and the balance of the host bits will give us 32 subnets. Classful addressing will enable us to design the network depending up on the number of subnets by taking bits from the hosts part.


HTH

[drago11]

I know how to subnet an ip address, but thanks for the example and help. Correct me if i'm wrong but, a classful address always has an 8, 16, or 24 bit network field, so by subnetting the way you did by borrowing host bit's is considered classless?

But anyway, my confusion is with the terminology. What does "The bits in between the network and host part comprise the third part" mean.

Odem says classful addresses have 3 parts: network, subnet and host. Classless have 2 parts: subnet or prefix and host.

[drago11]

Ok thanks for the help guys. Studying for a while today, simple things start getting to your head.

[sbjones]

Code: Select all

address = 172.16.0.0
mask     = 255.255.0.0   
mask in binary = 11111111.11111111.0000000.00000000
                 [    network    ] [    hosts     ]

address = 172.16.32.0
mask     =  255.255.224.0
mask in binay = 11111111.11111111.11100000.00000000
                [    network    ] ---[   hosts    ]

the 3 dashes --- are the subnet bits. They are the bits in between the hosts and the natural network.

[drago11]

Hey Sb,

Thank you sir. That just confirmed my thoughts exactly. I appreciate the help.