Classful Networks and CIDR Routing

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Before CIDR IP networking was classful. In 1981 three different sizes of networks were defined. Basically, due to an IP address shortage, the classful system was eventually replaced by a new system that is known as C.I.D.R. (Classless Inter-Domain Routing).

Classful Network IP Address Table

   bits                   addresses  Private IP Addresses
class    subnet mask           usable (-high,low,gw)     Private Last Address 
-------------------------------------------------------------------------------
   /32  255.255.255.255  1    
   /31  255.255.255.254  2    
   /30  255.255.255.252  4    1
   /29  255.255.255.248  8    5
   /28  255.255.255.240  16   13
   /27  255.255.255.224  32   29
   /26  255.255.255.192  64   61
   /25  255.255.255.128  128  125
.
C  /24  255.255.255.0    256  253
.
   /23  255.255.254.0    512
   /22  255.255.252.0    1024
   /21  255.255.248.0    2048
   /20  255.255.240.0    4096        172.16.0.0/20   ->  172.16.31.255
   /19  255.255.224.0    8192
   /18  255.255.192.0    16384
   /17  255.255.128.0    32768
.
B  /16  255.255.0.0      65536       192.168.0.0/16  ->  192.168.255.255
.
   /15  255.254.0.0      131072
   /14  255.252.0.0      262144
   /13  255.248.0.0      524288
   /12  255.240.0.0      1048576
   /11  255.224.0.0      2097152
   /10  255.192.0.0      4194304
   /9   255.128.0.0      8388608
.
A  /8   255.0.0.0        16777216    10.0.0.0/8      ->  10.255.255.255 
.
   /7   254.0.0.0        33554432
   /6   252.0.0.0        67108864
   /5   248.0.0.0        134217728
   /4   240.0.0.0        268435456
   /3   224.0.0.0        536870912
   /2   192.0.0.0        1073741824
   /1   128.0.0.0        2147483648
   /0   0.0.0.0          4294967296  

-------------------------------------------------------------------------------
Sat Oct 25 11:18:16 CDT 2003


6 IP Subnet division examples

. 6 IP Subnet division examples
. ------------------------------------------------------------------------------
.     bitnet: 192.168.0.0/30            -     bitnet: 192.168.0.0/29
.     subnet: 255.255.255.252           -     subnet: 255.255.255.248
.    network: 192.168.0.0               -    network: 192.168.0.0
.  broadcast: 192.168.0.3               -  broadcast: 192.168.0.7
.    gateway: 192.168.0.1               -    gateway: 192.168.0.1
.  complete range:                      - complete range:
.  192.168.0.0 - 192.168.0.3            - 192.168.0.0 - 192.168.0.7
.    usable range:                      -   usable range:
.  192.168.0.2                          - 192.168.0.2 - 192.168.0.6
. ------------------------------------------------------------------------------
.     bitnet: 192.168.0.64/26           -     bitnet: 10.10.4.128/25
.     subnet: 255.255.255.192           -     subnet: 255.255.255.128
.    network: 192.168.0.64              -    network: 10.10.4.128
.  broadcast: 192.168.0.127             -  broadcast: 10.10.4.255
.    gateway: 192.168.0.65              -    gateway: 10.10.4.254
.  complete range:                      - complete range:
.  192.168.0.64 - 192.168.0.127         -  10.10.4.128 - 10.10.4.255
.    usable range:                      -   usable range:
.  192.168.0.66 - 192.168.0.127         -  10.10.4.129 - 10.10.4.253
. ------------------------------------------------------------------------------
.     bitnet: 172.16.0.0/23             -     bitnet: 10.21.204.248/29
.     subnet: 255.255.254.0             -     subnet: 255.255.255.248
.    network: 172.16.0.0                -    network: 10.21.204.248
.  broadcast: 172.16.1.255              -  broadcast: 10.21.204.255
.    gateway: 172.16.1.254              -    gateway: 10.21.204.249
.  complete range:                      - complete range:
.  172.16.0.0 - 172.16.1.255            - 10.21.204.248 - 10.21.204.255
.    usable range:                      -   usable range:
.  172.16.0.1 - 172.16.1.253            - 10.21.204.250 - 10.21.204.254
. ------------------------------------------------------------------------------
. Sat Oct 25 18:16:15 CDT 2003


Classful subnet maps legal and illegal demonstrated

. Example subnet maps legal and illegal demonstrated
. ------------------------------------------------------------------------------
.
. [ scaled 24 bit network division to 28 bits ]
.
.                                  |
.                                  |
.                  |               |               |
.                  |               |               |
.          |       |       |       |       |       |       |
.          |       |       |       |       |       |       |    
.      |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
.   ...|...|...|...|...|...|...|...|...|...|...|...|...|...|...|...|
. 0   15  31  47  63  79  95  111 127 143 159 175 191 207 223 239 255
.  1   16  32  48  64  80  96  112 128 144 160 176 192 208 224 240 256
.
.
. [ legal 25 bit network division to 29 bits ]
.
.                                  |
.                                  |
.                  |               |               |
.                  |               |               | 
.                  |       |       |               |
.                  |       |       |               |          
.      |   |   |   |       |       |               |
.   ...|...|...|...|.......|.......|...............|...............|
.
.  1   8   16  24  32      48      64              96              128
.
.  four 29 bit subnets
.  two 28 bit subnets
.  two 27 bit subnets
.
. [ illegal 25 bit network division example ]
.
.                                  |
.                                  |
.                  |               |
.                  |               |
.                  |       |       |       |     
.                  |       |       |       |
.      |       |   |       |       |       |     
.   ...|.......|...|.......|.......|.......|.......................|
.
. 1    8       24  32      48      64      80                      128
.
.  this is a non-standard configuration
.
. ------------------------------------------------------------------------------
. Sat Oct 25 18:44:40 CDT 2003
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