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IPv4 CIDR Notation Reference

Complete reference table for IPv4 CIDR prefixes, subnet masks, and host counts. Quick lookup for network planning and subnet design.

ClassCIDRTotal HostsUsable HostsNetmaskWildcard Mask
/04,294,967,2964,294,967,2940.0.0.0255.255.255.255
/12,147,483,6482,147,483,646128.0.0.0127.255.255.255
/21,073,741,8241,073,741,822192.0.0.063.255.255.255
/3536,870,912536,870,910224.0.0.031.255.255.255
/4268,435,456268,435,454240.0.0.015.255.255.255
/5134,217,728134,217,726248.0.0.07.255.255.255
/667,108,86467,108,862252.0.0.03.255.255.255
/733,554,43233,554,430254.0.0.01.255.255.255
A/816,777,21616,777,214255.0.0.00.255.255.255
/98,388,6088,388,606255.128.0.00.127.255.255
/104,194,3044,194,302255.192.0.00.63.255.255
/112,097,1522,097,150255.224.0.00.31.255.255
/121,048,5761,048,574255.240.0.00.15.255.255
/13524,288524,286255.248.0.00.7.255.255
/14262,144262,142255.252.0.00.3.255.255
/15131,072131,070255.254.0.00.1.255.255
B/1665,53665,534255.255.0.00.0.255.255
/1732,76832,766255.255.128.00.0.127.255
/1816,38416,382255.255.192.00.0.63.255
/198,1928,190255.255.224.00.0.31.255
/204,0964,094255.255.240.00.0.15.255
/212,0482,046255.255.248.00.0.7.255
/221,0241,022255.255.252.00.0.3.255
/23512510255.255.254.00.0.1.255
C/24256254255.255.255.00.0.0.255
/25128126255.255.255.1280.0.0.127
/266462255.255.255.1920.0.0.63
/273230255.255.255.2240.0.0.31
/281614255.255.255.2400.0.0.15
/2986255.255.255.2480.0.0.7
/3042255.255.255.2520.0.0.3
/3122255.255.255.2540.0.0.1
/3211255.255.255.2550.0.0.0

About CIDR (Classless Inter-Domain Routing)

CIDR is a method for allocating IP addresses and routing that replaces the old classful network addressing architecture. CIDR notation uses a suffix (e.g., /24) to indicate the number of bits used for the network portion of the address.

How to Read CIDR Notation

The number after the slash (/) represents the network prefix length. For example, 192.168.1.0/24 means:

  • • The first 24 bits are the network portion
  • • The remaining 8 bits (32-24) are for host addresses
  • • This allows for 2^8 = 256 total IP addresses
  • • Usable hosts = 256 - 2 = 254 (excluding network and broadcast addresses)

Understanding the Table

  • • Class: Traditional network class (A, B, or C) - shown for reference only
  • • CIDR: The prefix length (/0 to /32)
  • • Total Hosts: All IP addresses in this subnet (2^(32-prefix))
  • • Usable Hosts: Total minus network and broadcast addresses
  • • Netmask: Subnet mask in dotted decimal notation
  • • Wildcard Mask: Inverse of the netmask (used in ACLs and routing)

Common Use Cases

  • • /8: Large networks (ISPs, major organizations) - 16.7M hosts
  • • /16: Medium-sized networks (enterprises) - 65K hosts
  • • /24: Small networks (offices, departments) - 254 usable hosts
  • • /30: Point-to-point links - 2 usable hosts
  • • /31: Point-to-point links (RFC 3021) - 2 usable hosts, no network/broadcast
  • • /32: Single host route

Private IP Address Ranges

  • • 10.0.0.0/8 (Class A): 10.0.0.0 - 10.255.255.255
  • • 172.16.0.0/12 (Class B): 172.16.0.0 - 172.31.255.255
  • • 192.168.0.0/16 (Class C): 192.168.0.0 - 192.168.255.255

Calculating Usable Hosts

For most networks: Usable Hosts = Total Hosts - 2

  • • Subtract 1 for the network address (first address)
  • • Subtract 1 for the broadcast address (last address)
  • • Example: /24 has 256 total addresses, 254 usable (256 - 2)
  • • Exception: /31 networks (RFC 3021) use all 2 addresses for point-to-point links
  • • Exception: /32 represents a single host (1 usable address)

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