What does the CIDR prefix number mean?

The CIDR prefix (also called the prefix length or subnet mask length) is the number after the slash in a notation like 192.168.1.0/24. It tells you how many of the 32 bits in the IP address are the network portion. A /24 means 24 bits are fixed for the network, leaving 8 bits for hosts (2^8 = 256 addresses, 254 usable). A /16 leaves 16 bits for hosts (65,536 addresses, 65,534 usable). The larger the prefix number, the smaller the subnet.

Why are there two fewer usable hosts than total addresses?

Every subnet reserves two addresses that cannot be assigned to hosts. The first address (all host bits set to 0) is the network address, used to identify the subnet itself. The last address (all host bits set to 1) is the broadcast address, used to send packets to all hosts on the subnet simultaneously. This is why a /24 subnet with 256 total addresses has only 254 usable host addresses. The exception is /31 subnets (used for point-to-point links per RFC 3021, with 2 usable addresses) and /32 host routes.

What is a wildcard mask and when is it used?

A wildcard mask is the bitwise inverse of the subnet mask. Where the subnet mask has 1 bits (fixed network bits), the wildcard mask has 0 bits, and vice versa. Wildcard masks are used in Cisco ACLs (access control lists) and OSPF network statements to specify which bits must match and which can vary. For example, a /24 subnet mask of 255.255.255.0 has a wildcard mask of 0.0.0.255, meaning the last octet can be any value.

IP Subnet Calculator (CIDR)

This calculator works out the key details of an IPv4 subnet from an IP address and a CIDR prefix length, the shorthand network engineers use for how many bits of an address are reserved for the network rather than individual hosts. Enter an IPv4 address such as 192.168.1.0, choose a prefix from /0 to /32, or type the full CIDR notation such as 192.168.1.0/24 and it fills in the rest automatically. The calculator returns the subnet mask, network address, broadcast address, wildcard mask, first and last usable host, and the total number of usable hosts in that range. Quick presets cover common private ranges under RFC 1918, and you can split a subnet into 2, 4, 8 or 16 equal sub-subnets to see how the prefix length and host ranges change. A colour-coded binary breakdown shows which bits are fixed for the network and which are free for hosts, handy when learning subnetting or checking a design. Use it to plan IP address allocation for a home network, office LAN or cloud VPC, to confirm a subnet is large enough for the devices you need, or to work out the wildcard mask for a Cisco access control list or OSPF network statement. All results are calculated instantly in your browser using standard binary arithmetic, based on RFC 950 and RFC 4632, and no data is sent to any server.

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Standard reference tool IPv4 subnetting per RFC 950 and CIDR per RFC 4632.

1. IP Address

IPv4 address (e.g. 192.168.1.0)

CIDR prefix length

Or enter full CIDR notation (overrides above)

2. Quick Presets

Common network ranges

Number of subnets to divide into (optional)

Subnet Details

Network Address
-
First address in subnet

Subnet Mask

Dotted decimal

Broadcast Address

Last address in subnet

First Usable Host

Lowest assignable IP

Last Usable Host

Highest assignable IP

Usable Hosts
-
Excludes network + broadcast

Full Subnet Summary

CIDR notation-

Network address-

Subnet mask-

Wildcard mask-

Broadcast address-

First usable host-

Last usable host-

Total addresses-

Usable hosts-

Subnet Division

Select a division above to split this subnet.

IP address: -

Subnet mask: -

Network address: -

Green = network bits (fixed) Amber = host bits (variable)

How IP Subnetting Works

Every IPv4 address is a 32-bit number, written as four octets (groups of 8 bits) separated by dots. The address is split into two parts: the network portion (which identifies the subnet) and the host portion (which identifies individual devices within the subnet). The subnet mask defines where this boundary falls. In CIDR (Classless Inter-Domain Routing) notation, the prefix length after the slash tells you how many bits belong to the network.

Core Formulas

Given an IPv4 address and a prefix length n:

Subnet mask: the leftmost n bits are all 1s; the remaining (32 - n) bits are all 0s.
Network address: IP address AND subnet mask (bitwise).
Broadcast address: network address OR (NOT subnet mask) (bitwise).
Wildcard mask: NOT subnet mask (bitwise) = 255.255.255.255 minus subnet mask.
Total addresses in block: 2^(32 - n).
Usable host addresses: 2^(32 - n) - 2 for prefix lengths 0 to 30. For /31, both addresses are usable (RFC 3021). For /32, there is 1 host (the address itself).
First host: network address + 1.
Last host: broadcast address - 1.

Worked Example: 192.168.1.0/24

The default values in this calculator use 192.168.1.0/24, the most common home and small office network range. Here is how the results are derived:

Field	Value	Working
Prefix length	/24	24 network bits, 8 host bits
Subnet mask	255.255.255.0	24 ones followed by 8 zeros in binary
Wildcard mask	0.0.0.255	Inverse of subnet mask
Network address	192.168.1.0	192.168.1.0 AND 255.255.255.0
Broadcast address	192.168.1.255	192.168.1.0 OR 0.0.0.255
First usable host	192.168.1.1	Network address + 1
Last usable host	192.168.1.254	Broadcast address - 1
Total addresses	256	2^8 = 256
Usable hosts	254	256 - 2 = 254

Common Prefix Lengths at a Glance

CIDR	Subnet Mask	Total Addresses	Usable Hosts
/8	255.0.0.0	16,777,216	16,777,214
/16	255.255.0.0	65,536	65,534
/20	255.255.240.0	4,096	4,094
/22	255.255.252.0	1,024	1,022
/24	255.255.255.0	256	254
/25	255.255.255.128	128	126
/26	255.255.255.192	64	62
/27	255.255.255.224	32	30
/28	255.255.255.240	16	14
/29	255.255.255.248	8	6
/30	255.255.255.252	4	2
/31	255.255.255.254	2	2 (RFC 3021)
/32	255.255.255.255	1	1 (host route)

Private IP Ranges (RFC 1918)

Three IPv4 ranges are reserved for private use and are not routed on the public internet. These are used for internal networks behind NAT (Network Address Translation):

10.0.0.0/8 (10.0.0.0 to 10.255.255.255) - 16,777,214 usable hosts
172.16.0.0/12 (172.16.0.0 to 172.31.255.255) - 1,048,574 usable hosts
192.168.0.0/16 (192.168.0.0 to 192.168.255.255) - 65,534 usable hosts

Related Calculators

Sources and method: IPv4 subnetting per RFC 950 (Internet Standard Subnetting Procedure). CIDR notation per RFC 4632 (Classless Inter-domain Routing). /31 point-to-point host addressing per RFC 3021. All calculations are performed in 32-bit unsigned integer arithmetic in the browser.

This tool handles IPv4 addresses only. IPv6 subnetting uses 128-bit addresses and different notation. All results are computed client-side and no data is transmitted to any server.