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IP/CIDR Reference

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This article aims to provide information about IP addresses, subnets, and CIDR notation.

What is an IP address?

In TCP/IP networking, an IP address is a value assigned to a device on a network for purposes of communication. There are currently two types of addresses, IPv4 (for example, and IPv6 (for example, 2001:0db8:85a3:0000:0000:8a2e:0370:7334).

  • IPv4 addresses are 32-bit integers represented in groups of four (4), separated by periods (.). Integers within each group range from 0 to 255. The IPv4 addressing scheme allows for a total of 232 (approximately 4 billion) addresses.
  • IPv6 addresses are 128-bit integers represented in hexadecimal, separated by colons (:).
For the purposes of this particular article, we will focus primarily on IPv4 addresses.

Reference Table

The following is reference table shows all valid CIDR lengths, their associated subnets, class, and supported capacity.

CIDR Length


# Networks

Host Capacity

/1128.0.0.0128 A2,147,483,392
/2192.0.0.064 A1,073,741,696
/3224.0.0.032 A536,870,848
/4240.0.0.016 A268,435,424
/5248.0.0.08 A134,217,712
/6252.0.0.04 A67,108,856
/7254.0.0.02 A33,554,428
/8255.0.0.01 A16,777,214
/9255.128.0.0128 B8,388,352
/10255.192.0.064 B4,194,176
/11255.224.0.032 B2,097,088
/12255.240.0.016 B1,048,544
/13255.248.0.08 B524,272
/14255.252.0.04 B262,136
/15255.254.0.02 B131,068
/16255.255.0.01 B65,024
/17255.255.128.0128 C32,512
/18255.255.192.064 C16,256
/19255.255.224.032 C8,128
/20255.255.240.016 C4,064
/21255.255.248.08 C2,032
/22255.255.252.04 C1,016
/23255.255.254.0 2 C508
/24255.255.255.01 C254
/25255.255.255.1282 subnets124
/26255.255.255.1924 subnets62
/27255.255.255.2248 subnets30
/28255.255.255.24016 subnets14
/29255.255.255.24832 subnets6
/30255.255.255.25264 subnets2
/32255.255.255.2551/256 C1

Reserved Addresses

The following is a table showing addresses that are officially reserved.








a single Class A network number


16 contiguous Class B network numbers


256 contiguous Class C network numbers

Special Credits

Special thanks to Ray Smith's website from 1996, located at www.rjsmith.com/CIDR-Table.html for the Reference Table and the majority of the information regarding CIDR that is found on this page.