CIDR stands for Classless Inter-Domain Routing. CIDR was developed in the 1990s as a standard scheme for routing network traffic across the Internet.


/27      1/8th of a Class C     32 hosts
/26      1/4th of a Class C     64 hosts
/25      1/2 of a Class C       128 hosts
/24      1 Class C              256 hosts
/23      2 Class C              512 hosts
/22      4 Class C              1,024 hosts
/21      8 Class C              2,048 hosts
/20      16 Class C             4,096 hosts
/19      32 Class C             8,192 hosts
/18      64 Class C             16,384 hosts
/17      128 Class C            32,768 hosts
/16      256 Class C            65,536 hosts (= 1 Class B)
/15      512 Class C            131,072 hosts
/14      1,024 Class C          262,144 hosts
/13      2,048 Class C          524,288 hosts

 
 

CIDR Notation

IPv4 CIDR
IP/CIDR? to last IP addrMaskHosts (*)ClassNotes
a.b.c.d/32+0.0.0.0255.255.255.25511/256 C
a.b.c.d/31+0.0.0.1255.255.255.25421/128 Cd = 0 … (2n) … 254
a.b.c.d/30+0.0.0.3255.255.255.25241/64 Cd = 0 … (4n) … 252
a.b.c.d/29+0.0.0.7255.255.255.24881/32 Cd = 0 … (8n) … 248
a.b.c.d/28+0.0.0.15255.255.255.240161/16 Cd = 0 … (16n) … 240
a.b.c.d/27+0.0.0.31255.255.255.224321/8 Cd = 0 … (32n) … 224
a.b.c.d/26+0.0.0.63255.255.255.192641/4 Cd = 0, 64, 128, 192
a.b.c.d/25+0.0.0.127255.255.255.1281281/2 Cd = 0, 128
a.b.c.0/24+0.0.0.255255.255.255.0002561 C
a.b.c.0/23+0.0.1.255255.255.254.0005122 Cc = 0 … (2n) … 254
a.b.c.0/22+0.0.3.255255.255.252.0001,0244 Cc = 0 … (4n) … 252
a.b.c.0/21+0.0.7.255255.255.248.0002,0488 Cc = 0 … (8n) … 248
a.b.c.0/20+0.0.15.255255.255.240.0004,09616 Cc = 0 … (16n) … 240
a.b.c.0/19+0.0.31.255255.255.224.0008,19232 Cc = 0 … (32n) … 224
a.b.c.0/18+0.0.63.255255.255.192.00016,38464 Cc = 0, 64, 128, 192
a.b.c.0/17+0.0.127.255255.255.128.00032,768128 Cc = 0, 128
a.b.0.0/16+0.0.255.255255.255.000.00065,536256 C = 1 B
a.b.0.0/15+0.1.255.255255.254.000.000131,0722 Bb = 0 … (2n) … 254
a.b.0.0/14+0.3.255.255255.252.000.000262,1444 Bb = 0 … (4n) … 252
a.b.0.0/13+0.7.255.255255.248.000.000524,2888 Bb = 0 … (8n) … 248
a.b.0.0/12+0.15.255.255255.240.000.0001,048,57616 Bb = 0 … (16n) … 240
a.b.0.0/11+0.31.255.255255.224.000.0002,097,15232 Bb = 0 … (32n) … 224
a.b.0.0/10+0.63.255.255255.192.000.0004,194,30464 Bb = 0, 64, 128, 192
a.b.0.0/9+0.127.255.255255.128.000.0008,388,608128 Bb = 0, 128
a.0.0.0/8+0.255.255.255255.000.000.00016,777,216256 B = 1 A
a.0.0.0/7+1.255.255.255254.000.000.00033,554,4322 Aa = 0 … (2n) … 254
a.0.0.0/6+3.255.255.255252.000.000.00067,108,8644 Aa = 0 … (4n) … 252
a.0.0.0/5+7.255.255.255248.000.000.000134,217,7288 Aa = 0 … (8n) … 248
a.0.0.0/4+15.255.255.255240.000.000.000268,435,45616 Aa = 0 … (16n) … 240
a.0.0.0/3+31.255.255.255224.000.000.000536,870,91232 Aa = 0 … (32n) … 224
a.0.0.0/2+63.255.255.255192.000.000.0001,073,741,82464 Aa = 0, 64, 128, 192
a.0.0.0/1+127.255.255.255128.000.000.0002,147,483,648128 Aa = 0, 128
0.0.0.0/0+255.255.255.255000.000.000.0004,294,967,296256 A

The link below is quite useful for creating CIDR entries:
http://bonomo.info/coyote/cidr-calculator.php