[Mishmash] Class of Address Part II

[GALEHALLOCK at aol.com]

In a message dated 07/16/2007 12:41:54 A.M. Eastern Daylight Time,  
fatkinson at mishmash.com writes:

   Here's the nuts and bolts of why IP  addresses fall into the different 
classes (as defined by all of the Internet  committees): 
 
    As you know, an IP address  consists of four octets such as #.#.#.# where 
each # represents an octet  (eight bits, also known as a byte).  
 
    Those of you that understand  binary know that eight bits can represent 
256 numbers (2 to the 8th power  equals 256).  The range is from 0 to 255.  
Thus each octet can be  represented by any number from 0 to 255 (with one 
exception that I'll mention  at the end).  
 
    In the decimal  system, 123 represents a number consisting of one one 
hundred, two tens,  and three ones.  Each column represents a different power of 
ten.   

In the binary system, each  column represents a power of two.  Example: 
00000011 represents a decimal  3 in binary because the last one represents a one 
and the next to the last  represents a 2.  
 
    00001111 represents decimal 15  because in addition to the 1 and the 2, 
the third from the end represents a 4,  and the fourth from the end represents 
an 8.  1+2+4+8=15.   

It works like this:  

 1   1    1   1   1   1   1    1
          -----------------------------------
                 1   6   3   1   8    4   2   1
         2   4    2   6
         8
 
    For each one, you add the  value.  In the case of 11111111, you'd add 
them like this: 
 
         128 
         +64
         +32
                 +16
                  +8
         +4
                  +2
         +1
               ------
                255
 
    For 10101010 you add it like  this: 
 
 
 128 
         +32
                  +8
                  +2
               ------

               170
 
    Remember that a computer can  only work in ones and zeroes.  That's why 
we have to convert to  binary.  
 
    We said that a Class A network  always begins with a number from 0 to 
127.  In binary, these numbers are  represented as: 
 
         00000000 binary = 0 decimal 
         01111111 binary = 127 decimal 
 
    Do you see a similarity  between the two binary values?  The first bit is 
a 0.  Hence  you have a Class A address.  For all Class A addresses, the 
first bit of  the first octet is always a 0.  This is because the Internet 
committees  have decided that this is what identifies an IP address as a Class A  
address.  These same committees made the decisions for the other classes  as 
well.  
 
    Class B's first octet can be  anywhere from 128 to 191.  They are 
represented in binary as:  

 10000000 binary = 128 decimal 
         10111111 binary = 191 decimal  
 
    Do you notice any similarity  in the binary values?  They both begin with 
10.  That makes  it a Class B address.  
 
    Class C address's first octet  must be from 192 to 223.  They are 
represented in binary as: 
 
         11000000 binary = 192 decimal 
         11011111 binary = 223 decimal 
 
    The similarity here is that  the first three bits are represented by 110. 
 
 
    There are Class D addresses  for special purposes.  Their first octet's 
binary representation must  begin with 1110.  For Class E (very limited use) 
they must begin  with 11110.  
 
    They don't go higher than  class E.  So the highest number in the first 
octet can be 11110111  binary = 247.  Hence, the first octet of an IP address 
can never be  higher than 247 (that is the exception that I mentioned earlier). 
  


Oh!

 
    I'm going to give a pop quiz  on this soon. ;-)

Oh god!!!!
 
    Any questions, feel free to  ask.
 
Too numerous to  list.
 

 
Gale
 
  
 
 
 
 

 



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