What does CIDR notation mean?

CIDR stands for Classless Inter-Domain Routing. It is a compact way to write an IP address and its subnet mask together. The prefix length (the number after the slash, such as /24) tells you how many leading bits are fixed as the network portion. For example, 192.168.1.0/24 means the first 24 bits identify the network, leaving 8 bits for host addresses, giving 256 total addresses (254 usable hosts). CIDR replaced the older classful system (Class A, B, C) to allow more flexible allocation of IP address space.

How many usable hosts does a /24 subnet have?

A /24 subnet has 2^(32-24) = 256 total IP addresses. Two of these are reserved: the network address (the first address) and the broadcast address (the last address). This leaves 254 usable host addresses. In general, any subnet has 2^(32-prefix) total addresses and 2^(32-prefix) minus 2 usable hosts. The exception is a /31 subnet, which RFC 3021 allows to use both addresses for point-to-point links, and a /32, which identifies a single host.

What is the difference between subnet mask and wildcard mask?

The subnet mask is written with 1-bits for the network portion and 0-bits for the host portion. For example, /24 gives a subnet mask of 255.255.255.0. The wildcard mask is the bitwise inverse of the subnet mask: 0-bits for the network portion and 1-bits for the host portion. A /24 wildcard mask is 0.0.0.255. Wildcard masks are used in access control lists (ACLs) on routers and firewalls, and in OSPF configurations, to specify which bits of an address to match.

CIDR Calculator

Enter an IP address and CIDR prefix length to calculate the network address, broadcast address, subnet mask, wildcard mask, usable host range, and total host count. Supports all IPv4 prefix lengths from /0 to /32.

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Reference tool Standard IPv4 CIDR arithmetic per RFC 4632. Results are mathematically exact.

1. IP Address

IP address (IPv4, e.g. 192.168.1.0)

Please enter a valid IPv4 address (four octets 0-255).

Prefix length (CIDR notation)

2. Or Enter CIDR Block

CIDR block (e.g. 192.168.1.0/24)

Please enter a valid CIDR block (e.g. 192.168.1.0/24).

Type a CIDR block above to auto-fill the IP and prefix fields, or use the fields on the left directly.

Common networks

Results for 192.168.1.0/24

Network Address
-
First address in block

Broadcast Address

Last address in block

Usable Hosts

Total minus network and broadcast

Total Addresses

2^(32 - prefix)

Subnet Details

CIDR notation-

Subnet mask-

Wildcard mask-

Network address-

First usable host-

Last usable host-

Broadcast address-

Usable hosts-

Binary Representation

IP address-

Subnet mask-

Network address-

Broadcast-

Network bits are fixed (shown by the mask). Host bits vary to form individual addresses within the subnet.

Common Prefix Reference

Prefix	Subnet Mask	Total Addresses	Usable Hosts	Class Equiv.
/8	255.0.0.0	16,777,216	16,777,214	Class A
/16	255.255.0.0	65,536	65,534	Class B
/24	255.255.255.0	256	254	Class C
/25	255.255.255.128	128	126	Half C
/26	255.255.255.192	64	62	Quarter C
/27	255.255.255.224	32	30	Eighth C
/28	255.255.255.240	16	14	Sixteenth C
/29	255.255.255.248	8	6	1/32 C
/30	255.255.255.252	4	2	Point-to-point
/31	255.255.255.254	2	2 (RFC 3021)	P2P only
/32	255.255.255.255	1	1 (host route)	Single host

How CIDR Works

CIDR (Classless Inter-Domain Routing) notation expresses an IPv4 address and its network prefix as a single string: the address, a slash, and the prefix length. The prefix length specifies how many of the 32 bits in the address identify the network. The remaining bits identify individual hosts within that network.

For example, 192.168.1.0/24 means the first 24 bits (192.168.1) are the network portion and the last 8 bits are the host portion. This gives 2^8 = 256 total addresses, of which 254 are usable (the network address 192.168.1.0 and broadcast address 192.168.1.255 are reserved).

Formulas Used

Given an IP address and prefix length n:

Total addresses: 2^(32 - n)
Subnet mask: 32 bits with the first n set to 1, the rest 0. Written as four decimal octets.
Wildcard mask: Bitwise inverse of the subnet mask (0s and 1s swapped).
Network address: IP address AND subnet mask (bitwise AND).
Broadcast address: Network address OR wildcard mask (bitwise OR).
First usable host: Network address + 1 (not applicable for /31 or /32).
Last usable host: Broadcast address - 1 (not applicable for /31 or /32).
Usable hosts: 2^(32 - n) - 2. For /31: 2 (per RFC 3021). For /32: 1 (single host route).

Worked Example: 192.168.1.0/24

With IP 192.168.1.0 and prefix /24:

Field	Value
Subnet mask	255.255.255.0
Wildcard mask	0.0.0.255
Network address	192.168.1.0
Broadcast address	192.168.1.255
First usable host	192.168.1.1
Last usable host	192.168.1.254
Total addresses	256 (2^8)
Usable hosts	254

The calculator above shows exactly these values when you use the default inputs.

Private IP Address Ranges (RFC 1918)

Three IPv4 address ranges are reserved for private use and are not routed on the public internet:

10.0.0.0/8: 10.0.0.0 to 10.255.255.255 (16,777,216 addresses)
172.16.0.0/12: 172.16.0.0 to 172.31.255.255 (1,048,576 addresses)
192.168.0.0/16: 192.168.0.0 to 192.168.255.255 (65,536 addresses)

Most home and office networks use addresses from these ranges. Network address translation (NAT) maps private addresses to one or more public IP addresses for internet access.

Subnetting: Splitting a Block

You can divide any CIDR block into smaller subnets by increasing the prefix length. Each additional bit doubles the number of subnets and halves the number of hosts per subnet. For example, a single /24 (256 addresses) can be split into two /25 subnets (128 addresses each), four /26 subnets (64 addresses each), and so on. This is used in network design to segment traffic, apply different security policies to different parts of a network, and make efficient use of IP address space.

Related Calculators

Essential Calculators: all computer, digital, and converter tools.
IP Subnet Calculator (CIDR): detailed subnetting with supernet and subnet split views.
Bandwidth Calculator: data transfer time and speed.
Hash Generator (MD5, SHA): generate checksums and hashes from text.
Base64 Encoder / Decoder: encode or decode Base64 strings.

Sources / method: RFC 4632 "Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan". RFC 1918 "Address Allocation for Private Internets". RFC 3021 "Using 31-Bit Prefixes on IPv4 Point-to-Point Links". All arithmetic is standard IPv4 bitwise operations.

This calculator covers IPv4 addresses only. IPv6 uses 128-bit addresses and different notation conventions. Results are mathematically exact for the inputs provided.