RFC 820 (rfc820) - Page 2 of 22

Assigned numbers

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RFC 820                                                     January 1983
                                                        Assigned Numbers
Network Numbers


                        ASSIGNED NETWORK NUMBERS

   The network numbers listed here are used as internet addresses by the
   Internet Protocol (IP) [33,62].  The IP uses a 32-bit address field
   and divides that address into a network part and a "rest" or local
   address part.  The division takes 3 forms or classes.

      The first type of address, or class A, has a 7-bit network number
      and a 24-bit local address.  The highest-order bit is set to 0.
      This allows 128 class A networks.

                           1                   2                   3   
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   NETWORK   |                Local Address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Class A Address

      The second type of address, class B, has a 14-bit network number
      and a 16-bit local address.  The two highest-order bits are set to
      1-0.  This allows 16,384 class B networks.

                           1                   2                   3   
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1 0|           NETWORK         |          Local Address        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Class B Address

      The third type of address, class C, has a 21-bit network number
      and a 8-bit local address.  The three highest-order bits are set
      to 1-0-0.  This allows 2,097,152 class C networks.

                           1                   2                   3   
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1 1 0|                    NETWORK              | Local Address |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Class C Address

   One commonly used notation for internet host addresses divides the
   32-bit address into four 8-bit fields and specifies the value of each
   field as a decimal number with the fields separated by periods.  This
   is called the "dotted decimal" notation.  For example, the internet
   address of ISIF in dotted decimal is 010.002.000.052, or 10.2.0.52.


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