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