CIDR(Classless Inter-Domain Routing)

/etc/netmasks

192.0.0.0 255.255.0.0
192.168.0.0 255.255.255.0
192.168.10.128 255.255.255.128

===============================

Netmasks Expanded (/24 through /32)
Netmask 255.255.255.0 /24 (11111111.11111111.11111111.00000000)
1 subnet
LOW IP HI IP
x.x.x.0 x.x.x.255

Netmask 255.255.255.128 /25 (11111111.11111111.11111111.10000000)
2 subnets
LOW IP HI IP
x.x.x.0 x.x.x.127
x.x.x.128 x.x.x.255

Netmask 255.255.255.192 /26 (11111111.11111111.11111111.11000000)
4 subnets
x.x.x.0 x.x.x.63
x.x.x.64 x.x.x.127
x.x.x.128 x.x.x.191
x.x.x.192 x.x.x.255

Netmask 255.255.255.224 /27 (11111111.11111111.11111111.11100000)
8 subnets
x.x.x.0 x.x.x.31
x.x.x.32 x.x.x.63
x.x.x.64 x.x.x.95
x.x.x.96 x.x.x.127
x.x.x.128 x.x.x.159
x.x.x.160 x.x.x.191
x.x.x.192 x.x.x.223
x.x.x.224 x.x.x.255

Netmask 255.255.255.240 /28 (11111111.11111111.11111111.11110000)
16 subnets
x.x.x.0 x.x.x.15
x.x.x.16 x.x.x.31
x.x.x.32 x.x.x.47
x.x.x.48 x.x.x.63
x.x.x.64 x.x.x.79
x.x.x.80 x.x.x.95
x.x.x.96 x.x.x.111
x.x.x.112 x.x.x.127
x.x.x.128 x.x.x.143
x.x.x.144 x.x.x.159
x.x.x.160 x.x.x.175
x.x.x.176 x.x.x.191
x.x.x.192 x.x.x.207
x.x.x.208 x.x.x.223
x.x.x.224 x.x.x.239
x.x.x.240 x.x.x.255

Netmask 255.255.255.248 /29 (11111111.11111111.11111111.11111000)
32 subnets
x.x.x.0 x.x.x.7
x.x.x.8 x.x.x.15
x.x.x.16 x.x.x.23
x.x.x.24 x.x.x.31
x.x.x.32 x.x.x.39
x.x.x.40 x.x.x.47
x.x.x.48 x.x.x.55
x.x.x.56 x.x.x.63
x.x.x.64 x.x.x.71
x.x.x.72 x.x.x.79
x.x.x.80 x.x.x.87
x.x.x.88 x.x.x.95
x.x.x.96 x.x.x.103
x.x.x.104 x.x.x.111
x.x.x.112 x.x.x.119
x.x.x.120 x.x.x.127
x.x.x.128 x.x.x.135
x.x.x.136 x.x.x.143
x.x.x.144 x.x.x.151
x.x.x.152 x.x.x.159
x.x.x.160 x.x.x.167
x.x.x.168 x.x.x.175
x.x.x.176 x.x.x.183
x.x.x.184 x.x.x.191
x.x.x.192 x.x.x.199
x.x.x.200 x.x.x.207
x.x.x.208 x.x.x.215
x.x.x.216 x.x.x.223
x.x.x.224 x.x.x.231
x.x.x.232 x.x.x.239
x.x.x.240 x.x.x.247
x.x.x.248 x.x.x.255

Netmask 255.255.255.252 /30 (11111111.11111111.11111111.11111100)
64 subnets
LOW IP HI IP
x.x.x.0 x.x.x.3
x.x.x.4 x.x.x.7
x.x.x.8 x.x.x.11
x.x.x.12 x.x.x.15
x.x.x.16 x.x.x.19
x.x.x.20 x.x.x.23
x.x.x.24 x.x.x.27
x.x.x.28 x.x.x.31
x.x.x.32 x.x.x.35
x.x.x.36 x.x.x.39
x.x.x.40 x.x.x.43
x.x.x.44 x.x.x.47
x.x.x.48 x.x.x.51
x.x.x.52 x.x.x.55
x.x.x.56 x.x.x.59
x.x.x.60 x.x.x.63
x.x.x.64 x.x.x.67
x.x.x.68 x.x.x.71
x.x.x.72 x.x.x.75
x.x.x.76 x.x.x.79
x.x.x.80 x.x.x.83
x.x.x.84 x.x.x.87
x.x.x.88 x.x.x.91
x.x.x.92 x.x.x.95
x.x.x.96 x.x.x.99
x.x.x.100 x.x.x.103
x.x.x.104 x.x.x.107
x.x.x.108 x.x.x.111
x.x.x.112 x.x.x.115
x.x.x.116 x.x.x.119
x.x.x.120 x.x.x.123
x.x.x.124 x.x.x.127
x.x.x.128 x.x.x.131
x.x.x.132 x.x.x.135
x.x.x.136 x.x.x.139
x.x.x.140 x.x.x.143
x.x.x.144 x.x.x.147
x.x.x.148 x.x.x.151
x.x.x.152 x.x.x.155
x.x.x.156 x.x.x.159
x.x.x.160 x.x.x.163
x.x.x.164 x.x.x.167
x.x.x.168 x.x.x.171
x.x.x.172 x.x.x.175
x.x.x.176 x.x.x.179
x.x.x.180 x.x.x.183
x.x.x.184 x.x.x.187
x.x.x.188 x.x.x.191
x.x.x.192 x.x.x.195
x.x.x.196 x.x.x.199
x.x.x.200 x.x.x.203
x.x.x.204 x.x.x.207
x.x.x.208 x.x.x.211
x.x.x.212 x.x.x.215
x.x.x.216 x.x.x.219
x.x.x.220 x.x.x.223
x.x.x.224 x.x.x.227
x.x.x.228 x.x.x.231
x.x.x.232 x.x.x.235
x.x.x.236 x.x.x.239
x.x.x.240 x.x.x.243
x.x.x.244 x.x.x.247
x.x.x.248 x.x.x.251
x.x.x.252 x.x.x.255

net mask:

1111 1100 == 252

--------------------------------------------------------------------------------

Pozar's two-bit(tm) addressing

4-bit m m m m
2-bit m m
(.1) 0 0 0 0 0 0 0 1 (.2) 0 0 0 0 0 0 1 0
(.17) 0 0 0 1 0 0 0 1 (.18) 0 0 0 1 0 0 1 0
(.33) 0 0 1 0 0 0 0 1 (.34) 0 0 1 0 0 0 1 0
(.49) 0 0 1 1 0 0 0 1 (.50) 0 0 1 1 0 0 1 0
(.65) 0 1 0 0 0 0 0 1 (.66) 0 1 0 0 0 0 1 0
(.129) 1 0 0 0 0 0 0 1 (.130) 1 0 0 0 0 0 1 0
(.193) 1 1 0 0 0 0 0 1 (.194) 1 1 0 0 0 0 1 0
(.225) 1 1 1 0 0 0 0 1 (.226) 1 1 1 0 0 0 1 0

--------------------------------------------------------------------------------

Younker's tables

Here's a table showing the relationship between the / notation, the byte
notation, and the corresponding binary numbers (with a dot every eight
digits) for the 32 bit addresses. I've thrown in a count of how many
Class A/B/C networks the larger networks encompass.

/ Notation Binary Byte Notation #Class
---------- ----------------------------------- -------------- ------
/0 00000000.00000000.00000000.00000000 0.0.0.0 256 A
/1 10000000.00000000.00000000.00000000 128.0.0.0 128 A
/2 11000000.00000000.00000000.00000000 192.0.0.0 64 A
/3 11100000.00000000.00000000.00000000 224.0.0.0 32 A
/4 11110000.00000000.00000000.00000000 240.0.0.0 16 A
/5 11111000.00000000.00000000.00000000 248.0.0.0 8 A
/6 11111100.00000000.00000000.00000000 252.0.0.0 4 A
/7 11111110.00000000.00000000.00000000 254.0.0.0 2 A
/8 11111111.00000000.00000000.00000000 255.0.0.0 1 A
/9 11111111.10000000.00000000.00000000 255.128.0.0 128 B
/10 11111111.11000000.00000000.00000000 255.192.0.0 64 B
/11 11111111.11100000.00000000.00000000 255.224.0.0 32 B
/12 11111111.11110000.00000000.00000000 255.240.0.0 16 B
/13 11111111.11111000.00000000.00000000 255.248.0.0 8 B
/14 11111111.11111100.00000000.00000000 255.252.0.0 4 B
/15 11111111.11111110.00000000.00000000 255.254.0.0 2 B
/16 11111111.11111111.00000000.00000000 255.255.0.0 1 B
/17 11111111.11111111.10000000.00000000 255.255.128.0 128 C
/18 11111111.11111111.11000000.00000000 255.255.192.0 64 C
/19 11111111.11111111.11100000.00000000 255.255.224.0 32 C
/20 11111111.11111111.11110000.00000000 255.255.240.0 16 C
/21 11111111.11111111.11111000.00000000 255.255.248.0 8 C
/22 11111111.11111111.11111100.00000000 255.255.252.0 4 C
/23 11111111.11111111.11111110.00000000 255.255.254.0 2 C
/24 11111111.11111111.11111111.00000000 255.255.255.0 1 C
/25 11111111.11111111.11111111.10000000 255.255.255.128
/26 11111111.11111111.11111111.11000000 255.255.255.192
/27 11111111.11111111.11111111.11100000 255.255.255.224
/28 11111111.11111111.11111111.11110000 255.255.255.240
/29 11111111.11111111.11111111.11111000 255.255.255.248
/30 11111111.11111111.11111111.11111100 255.255.255.252
/31 11111111.11111111.11111111.11111110 255.255.255.254
/32 11111111.11111111.11111111.11111111 255.255.255.255

Here's an example of how to get from the binary number 11000000 to
the decimal number (192).

11000000 => 128*1 + 64*1 + 32*0 + 16*0 + 8*0 + 4*0 + 2*0 + 1*0
= 128 + 64 + 0 + 0 + 0 + 0 + 0 + 0
= 128 + 64
= 192

Another example (using an arbitrarily chosen binary number):

10000100 => 128*1 + 64*0 + 32*0 + 16*0 + 8*0 + 4*1 + 2*0 + 1*0
= 128 + 0 + 0 + 0 + 0 + 4 + 0 + 0
= 128 + 4
= 132