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CIDR Cheat Sheet

Complete IPv4 CIDR reference — subnet masks, wildcard masks, host counts, and binary breakdown from /0 to /32.

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IPv4 CIDR Reference Table

Every CIDR prefix from /0 to /32 with subnet mask, wildcard mask, total addresses, usable hosts, and binary representation. Bookmark this page for quick lookups.

Complete IPv4 CIDR reference — subnet mask, wildcard mask, total addresses, usable hosts, and binary subnet mask from /0 to /32
CIDRSubnet MaskWildcard MaskTotalUsableBinary
/00.0.0.0255.255.255.2554,294,967,2964,294,967,29400000000.00000000.00000000.00000000
/1128.0.0.0127.255.255.2552,147,483,6482,147,483,64610000000.00000000.00000000.00000000
/2192.0.0.063.255.255.2551,073,741,8241,073,741,82211000000.00000000.00000000.00000000
/3224.0.0.031.255.255.255536,870,912536,870,91011100000.00000000.00000000.00000000
/4240.0.0.015.255.255.255268,435,456268,435,45411110000.00000000.00000000.00000000
/5248.0.0.07.255.255.255134,217,728134,217,72611111000.00000000.00000000.00000000
/6252.0.0.03.255.255.25567,108,86467,108,86211111100.00000000.00000000.00000000
/7254.0.0.01.255.255.25533,554,43233,554,43011111110.00000000.00000000.00000000
/8255.0.0.00.255.255.25516,777,21616,777,21411111111.00000000.00000000.00000000
/9255.128.0.00.127.255.2558,388,6088,388,60611111111.10000000.00000000.00000000
/10255.192.0.00.63.255.2554,194,3044,194,30211111111.11000000.00000000.00000000
/11255.224.0.00.31.255.2552,097,1522,097,15011111111.11100000.00000000.00000000
/12255.240.0.00.15.255.2551,048,5761,048,57411111111.11110000.00000000.00000000
/13255.248.0.00.7.255.255524,288524,28611111111.11111000.00000000.00000000
/14255.252.0.00.3.255.255262,144262,14211111111.11111100.00000000.00000000
/15255.254.0.00.1.255.255131,072131,07011111111.11111110.00000000.00000000
/16255.255.0.00.0.255.25565,53665,53411111111.11111111.00000000.00000000
/17255.255.128.00.0.127.25532,76832,76611111111.11111111.10000000.00000000
/18255.255.192.00.0.63.25516,38416,38211111111.11111111.11000000.00000000
/19255.255.224.00.0.31.2558,1928,19011111111.11111111.11100000.00000000
/20255.255.240.00.0.15.2554,0964,09411111111.11111111.11110000.00000000
/21255.255.248.00.0.7.2552,0482,04611111111.11111111.11111000.00000000
/22255.255.252.00.0.3.2551,0241,02211111111.11111111.11111100.00000000
/23255.255.254.00.0.1.25551251011111111.11111111.11111110.00000000
/24255.255.255.00.0.0.25525625411111111.11111111.11111111.00000000
/25255.255.255.1280.0.0.12712812611111111.11111111.11111111.10000000
/26255.255.255.1920.0.0.63646211111111.11111111.11111111.11000000
/27255.255.255.2240.0.0.31323011111111.11111111.11111111.11100000
/28255.255.255.2400.0.0.15161411111111.11111111.11111111.11110000
/29255.255.255.2480.0.0.78611111111.11111111.11111111.11111000
/30255.255.255.2520.0.0.34211111111.11111111.11111111.11111100
/31255.255.255.2540.0.0.122*11111111.11111111.11111111.11111110
/32255.255.255.2550.0.0.011*11111111.11111111.11111111.11111111

* /31 uses both addresses as hosts (RFC 3021). /32 is a single host route.

How to Read This Table

CIDR prefix length (the number after the slash) specifies how many bits of the 32-bit IPv4 address identify the network. The remaining bits identify individual hosts within that network.

The subnet mask is the dotted-decimal representation of the prefix — each bit set to 1 in the prefix becomes 255, 254, 252, 248, 240, 224, 192, 128, or 0 in the corresponding octet. The wildcard mask is the bitwise inverse, commonly used in Cisco ACLs and OSPF configuration.

Total addresses equals 2(32 − prefix). Usable hostsis normally total − 2 (reserving the network address and broadcast address), but /31 networks (RFC 3021) use both addresses as hosts for point-to-point links, and /32 represents a single host route.

The binary column shows the subnet mask as 32 bits, with dots separating each octet. This makes the prefix boundary visually clear — the 1-bits on the left define the network, the 0-bits on the right define the hosts.

Private IPv4 Ranges (RFC 1918)

These address ranges are reserved for internal use and are not routable on the public internet:

  • 10.0.0.0/810.0.0.0 – 10.255.255.255 (16.7 million addresses)
  • 172.16.0.0/12172.16.0.0 – 172.31.255.255 (1 million addresses)
  • 192.168.0.0/16192.168.0.0 – 192.168.255.255 (65,536 addresses)

Frequently Asked Questions

What is CIDR notation?
CIDR (Classless Inter-Domain Routing) notation expresses an IP network as an address followed by a slash and a prefix length — for example, 192.168.1.0/24. The prefix length defines how many leading bits identify the network, replacing the older classful A/B/C system. CIDR enables flexible subnet sizes and route aggregation (supernetting), which is essential for efficient internet routing.
What is the difference between a subnet mask and a wildcard mask?
A subnet mask uses 1-bits to mark the network portion and 0-bits for the host portion (e.g. 255.255.255.0). A wildcard mask is the bitwise inverse — 0-bits for the network and 1-bits for the hosts (e.g. 0.0.0.255). Wildcard masks are used in Cisco access control lists (ACLs) and OSPF area definitions because they offer more flexibility than subnet masks for matching address ranges.
How do I calculate usable hosts from a CIDR prefix?
The formula is 2^(32 − prefix) − 2. For a /24: 2^8 − 2 = 254 usable hosts. The two subtracted addresses are the network address (all host bits zero) and the broadcast address (all host bits one). The exceptions are /31 (RFC 3021, 2 usable hosts with no reserved addresses) and /32 (1 host, a single route).
What is the smallest practical subnet?
A /30 subnet (4 addresses, 2 usable) is the traditional minimum for WAN point-to-point links. RFC 3021 introduced /31 specifically for point-to-point links — both addresses are usable hosts with no network or broadcast reserved, making /31 the smallest practical subnet. A /32 is not a subnet but a single host route, commonly used in routing tables and firewall rules.
Why does /24 have 254 usable hosts, not 256?
A /24 subnet contains 256 total addresses (2^8). Two are reserved: the network address (x.x.x.0, where all host bits are 0) identifies the subnet itself, and the broadcast address (x.x.x.255, where all host bits are 1) is used to send traffic to every host on the subnet. Neither can be assigned to an individual device, leaving 254 usable addresses.
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