subnetting / Class A,B, and C ip address vs. Class A,B, and C subnet mask

TECH929

Hello to TechExams members .    Tech929  here .    Newby.

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Please forgive this basic question.    But, felt it warranted some clarification .

As we all know:  IP Addresses fall into catagories of  Class A,B,C, & D    -------------   as well as :   subnet mask Classes    A,B,C, and D

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As i am practicing solving various subnetting problems  /  deciphering network address, broadcast address,  # of subnets,  # of host per subnet ,  1st usable ip  ,   last usable ip    etc,  etc.  .......

My question is :   Why do some of these problems I'm attempting to solve have a  ' specific Class ip address ;  B for example  '   and yet I'm given a class C subnet mask to work with   ?

I fail to see the correlation.      Why are problems not strictly based upon using a definitive Class B  ip add  ( for ex. )  with a Class B subnet mask  ?

What governs ,    or allows the combination ( mixing )  of these classes  /   between ip address and subnet mask   ?

Why do we not just use  Class A  ip address   with  Class A  subnet masks   ?

Why do we not just use  Class B  ip address   with  Class B  subnet masks   ?

Why do we not just use  Class C  ip address   with  Class C  subnet masks   ?

Thank you .

shahlapirnia

@TECH929

So, there is classful addressing and classless addressing.

Classful:

- Classes A, B, C, and D; E is reserved as experimental
- Subnet mask remains the same for each class

Classless:

- Classless Inter Domain Routing (CIDR)
- Variable Length Subnet Mask (VLSM)

Classless was introduced in order to better utilize the available IP address range given to a company.

TECH929

Hello  Shahlapirnia

Thank you for explaining classful vs classless   +   how the subnet remains the same within classful

vs

classless  ( contains  CIDR  +  VLSM  )

as well as the fact that classless was introduced to better utilize the available ip addresses [ range ] within a business.  

Very helpful .    And I thank you for your time and response !

Sincerely, 

alan