| What do "/32" or "/24" mean after an IP address? (CIDR) | |
| The number after the slash refers to
the significant
digit in a 32-bit byte. So, "/1" would refer to the most
significant digit (the one on the left) and "/32" refers to
the least significant digit (the one on the right). Classless Internet Domain Routing (CIDR) replaced the traditional "A-Class", "B-Class", and "C-Class" networks with notation such as: A-Class | 1.0.0.0/8 == 1.0.0.0 - 1.255.255.255 (1,703,936 IPs) B-Class | 1.2.0.0/16 == 1.2.0.0 - 1.2.255.255 (65,536 IPs) C-Class | 1.2.3.0/24 == 1.2.3.0 - 1.2.3.255 (256 IPs)Think about binary or "base-2" numbers, and positional notation or place-value notation. The "/n" part refers to the significant bit in a 32-bit byte which defines the subnet. So, a "/32" is the thirty-second most significant bit, or a single IP. A "/31" is twice that, a "/30" is twice a "/31", and a "/29" is 8 contiguous IP addresses. CIDR subnets fall on natural boundaries which are mathematical exponents of 2, so they can only start on certain IP numbers. For example, a CIDR subnet cannot be 1.2.3.10/27 because 10 is not an exponential value of 2. Here's another way to think about it. Looking at the four binary eight-bit bytes (octets) of an IP address, count the bits, starting from the left, until you find the significant bit (1) designating the address range: 00000000.00000000.00000000.00000001 /32
00000000.00000000.00000001.00000000 /24
00000000.00000001.00000000.00000000 /16
00000001.00000000.00000000.00000000 /8
^^^^^^^ ^ ^ ^
12345678 16 24 32
This is a very brief explanation, not intended for a complete
understanding of the math, nomenclature, or history of IP allocation.
For more information, try these pages:
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Article Source: www.spamhaus.org |
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Last Edited: 12/22/11
© 2011
Plant City Data Service