IP Addressing

CIDR Table

CIDR reference table from /0 to /32 with mask, wildcard, number of addresses, usable hosts, hex and class. Filter live by prefix or by mask. Includes the reserved and private ranges worth keeping at hand. Nothing is sent to any server.

redzilla.cl — cidr

CIDR /24

Mask 255.255.255.0

Wildcard 0.0.0.255

Hex 0xffffff00

CIDR table · 33 rows

CIDR	Mask	Wildcard	Addresses	Hosts	Hex	Class

Reserved and private ranges

Block	Addresses	Use	Reference

How each column is computed

1. The mask has n bits set to 1 followed by 32 − n bits set to 0. The wildcard is its complement (the inverted bits).

2. number of addresses = 2^(32 − n). It counts the whole network, including network and broadcast.

3. usable hosts = 2^(32 − n) − 2 (network and broadcast are subtracted) for n ≤ 30. On /31 it is 2 (RFC 3021, point-to-point links) and on /32 it is 1 (single host).

4. The class is a guide only: classful addressing was deprecated by CIDR, but /8, /16 and /24 echo classes A, B and C.

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How it works

It is a reference table with all 33 IPv4 prefixes, from /0 to /32. For each one it shows the dotted-decimal mask, the wildcard (the inverted mask used by Cisco ACLs and OSPF), the total number of addresses, the usable hosts, the mask in hexadecimal and the approximate historical class. You can filter live by typing a prefix (/24 or 24), a mask or a wildcard, and clicking a row gives you its full record ready to copy.

Every column comes from the binary arithmetic of the prefix: the mask has n ones followed by zeros, the total address count is 2^(32−n) and the usable hosts are 2^(32−n) − 2 (subtracting network and broadcast), with the exceptions of /31 (2 hosts, point-to-point links per RFC 3021) and /32 (1 host). It also includes the table of reserved and private ranges: RFC 1918, CGNAT (RFC 6598), loopback, link-local and multicast.

Example: reading the /26 row

Mask: 26 ones → 255.255.255.192; wildcard: the remaining 6 bits → 0.0.0.63.
Total addresses: 2^(32−26) = 64.
Usable hosts: 64 − 2 = 62; in hexadecimal the mask is 0xffffffc0.

Frequently asked questions

What mask corresponds to a /27, /28 or /29?

A /27 is 255.255.255.224 (30 usable hosts), a /28 is 255.255.255.240 (14 hosts) and a /29 is 255.255.255.248 (6 hosts). The quick rule: each extra prefix bit halves the block, so going from /27 to /28 drops you from 32 to 16 total addresses.

What is the difference between total addresses and usable hosts?

The total 2^(32−n) counts every address in the block, including the network address and the broadcast, which by convention are not assigned to devices. Usable hosts are that total minus 2. The exceptions are /31, which uses both of its addresses on point-to-point links (RFC 3021), and /32, which represents a single host.

Which IP ranges are private and which should never be routed to the internet?

The three RFC 1918 blocks: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. It also pays to know 100.64.0.0/10 (RFC 6598 CGNAT, used by ISPs and unsuitable for a LAN when there is CGNAT upstream), 127.0.0.0/8 (loopback), 169.254.0.0/16 (link-local/APIPA) and 224.0.0.0/4 (multicast). The table lists them with their reference RFC.

Do classes A, B and C still exist?

As an allocation mechanism, no: classful addressing became obsolete with CIDR in 1993 (RFC 1519). But the terms survive as a memory shortcut: /8 maps to class A, /16 to B and /24 to C. The class column in the table is for orientation only.

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Disclaimer

We take great care to keep every tool accurate and review it thoroughly; even so, we can't guarantee it is free of errors or take responsibility for how the results are used. We recommend double-checking anything critical.

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