There are the basic A,B,C class networks :
1st address Last address Netmask per network A: 0.0.0.0 ... 127.255.255.255 255.0.0.0 B: 128.0.0.0 ... 191.255.255.255 255.255.0.0 C: 192.0.0.0 ... 223.255.255.255 255.255.255.0
CIDR instead of limiting the network size to 245, 64k or 16M it uses any power of 2 for the network size. It sees the netmask as a 32 bit number instead of 4 bytes.
The CIDR (Classless Inter-Domain Routing) is the number of 1 from left to right in the netmask.
11111111 11111111 11111111 00000000 = 255.255.255.0 = /24 = 256 ips in the network (2^8) 32-24=8 bits
11111111 11111111 11111100 00000000 = 255.255.252.0 = /22 = 1024 ips in the network (2^10) 32-22=10 bits
Netmask: 255.255.255.252 or /30
Starts with multiple of 4
0 - 3 | 4-7 | 8-11 | 12-15 | ..... | 252-255
Binary Hex Quad Dec 2ⁿ CIDR Number of addresses 00000000000000000000000000000000 00000000 0.0.0.0 2³² /0 4,294,967,296 4 G 10000000000000000000000000000000 80000000 128.0.0.0 2³¹ /1 2,147,483,648 2 G 11000000000000000000000000000000 C0000000 192.0.0.0 2³⁰ /2 1,073,741,824 1 G 11100000000000000000000000000000 E0000000 224.0.0.0 2²⁹ /3 536,870,912 512 M 11110000000000000000000000000000 F0000000 240.0.0.0 2²⁸ /4 268,435,456 256 M 11111000000000000000000000000000 F8000000 248.0.0.0 2²⁷ /5 134,217,728 128 M 11111100000000000000000000000000 FC000000 252.0.0.0 2²⁶ /6 67,108,864 64 M 11111110000000000000000000000000 FE000000 254.0.0.0 2²⁵ /7 33,554,432 32 M 11111111000000000000000000000000 FF000000 255.0.0.0 2²⁴ /8 16,777,216 16 M 11111111100000000000000000000000 FF800000 255.128.0.0 2²³ /9 8,388,608 8 M 11111111110000000000000000000000 FFC00000 255.192.0.0 2²² /10 4,194,304 4 M 11111111111000000000000000000000 FFE00000 255.224.0.0 2²¹ /11 2,097,152 2 M 11111111111100000000000000000000 FFF00000 255.240.0.0 2²⁰ /12 1,048,576 1 M 11111111111110000000000000000000 FFF80000 255.248.0.0 2¹⁹ /13 524,288 512 k 11111111111111000000000000000000 FFFC0000 255.252.0.0 2¹⁸ /14 262,144 256 k 11111111111111100000000000000000 FFFE0000 255.254.0.0 2¹⁷ /15 131,072 128 k 11111111111111110000000000000000 FFFF0000 255.255.0.0 2¹⁶ /16 65,536 64 k 11111111111111111000000000000000 FFFF8000 255.255.128.0 2¹⁵ /17 32,768 32 k 11111111111111111100000000000000 FFFFC000 255.255.192.0 2¹⁴ /18 16,384 16 k 11111111111111111110000000000000 FFFFE000 255.255.224.0 2¹³ /19 8,192 8 k 11111111111111111111000000000000 FFFFF000 255.255.240.0 2¹² /20 4,096 4 k 11111111111111111111100000000000 FFFFF800 255.255.248.0 2¹¹ /21 2,048 2 k 11111111111111111111110000000000 FFFFFC00 255.255.252.0 2¹⁰ /22 1,024 1 k 11111111111111111111111000000000 FFFFFE00 255.255.254.0 2⁹ /23 512 11111111111111111111111100000000 FFFFFF00 255.255.255.0 2⁸ /24 256 11111111111111111111111110000000 FFFFFF80 255.255.255.128 2⁷ /25 128 11111111111111111111111111000000 FFFFFFC0 255.255.255.192 2⁶ /26 64 11111111111111111111111111100000 FFFFFFE0 255.255.255.224 2⁵ /27 32 11111111111111111111111111110000 FFFFFFF0 255.255.255.240 2⁴ /28 16 11111111111111111111111111111000 FFFFFFF8 255.255.255.248 2³ /29 8 11111111111111111111111111111100 FFFFFFFC 255.255.255.252 2² /30 4 11111111111111111111111111111110 FFFFFFFE 255.255.255.254 2¹ /31 2 11111111111111111111111111111111 FFFFFFFF 255.255.255.255 2⁰ /32 1
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