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IP Subnet Calculator: Free IPv4 CIDR Calculator

This free IP subnet calculator works out the network address, broadcast address, host range, subnet mask and wildcard mask for any IPv4 block. You can also split a network into smaller subnets. It runs in your browser.

By Debmedia Technologies LLP · Kolkata, India · Updated

What Is an IP Subnet Calculator?

An IP subnet calculator works out the details of an IPv4 network from an address and a prefix length or subnet mask. It tells you the network address, the broadcast address, the first and last usable host and how many hosts fit. Network engineers use these numbers daily to plan addresses and write firewall and routing rules.

Doing the binary maths by hand is slow and easy to get wrong. This calculator gives the answer at once and can also split a network into smaller subnets.

How to Use This IP Subnet Calculator

  1. Enter an IPv4 address with a prefix, such as 192.168.1.10/24, or with a mask, such as 192.168.1.10 255.255.255.0.
  2. Optionally enter a larger prefix in the split box to divide the network into subnets.
  3. Press Calculate.
  4. Read the subnet details and, if you asked for a split, the list of smaller subnets.

Common Prefix Lengths

CIDRSubnet maskUsable hosts
/8255.0.0.016,777,214
/16255.255.0.065,534
/24255.255.255.0254
/26255.255.255.19262
/28255.255.255.24014
/30255.255.255.2522
/32255.255.255.2551 (a single host)

Classless addressing and the prefix notation are described in RFC 4632, classless inter-domain routing.

Terms Explained

  • Network address: the first address of the block. It names the subnet and is not given to a host.
  • Broadcast address: the last address. A packet sent to it reaches every host in the subnet.
  • Usable hosts: the addresses between the two, which is total addresses minus two.
  • Wildcard mask: the inverse of the subnet mask, used in router access lists.
  • Address type: whether the block is private, loopback, link-local or public.

Two special cases exist. A /31 has two usable addresses for point-to-point links, and a /32 is a single host.

Planning Subnets

To divide a network, pick a larger prefix. A /24 split into /26 gives four subnets of 62 hosts each. Always leave room to grow: choose subnets that are about twice as large as your current need. For the IPv6 side of planning, see the IPv6 prefix calculator, and to see an address as a number use the IP to number converter. You can also check who owns a public block with the WHOIS owner lookup.

Quick Rules of Thumb

Each extra prefix bit halves the number of addresses, so a /25 holds half a /24. Add 1 to the last octet of the mask to know the block size: a mask ending in 192 means blocks of 64. These shortcuts help you check the calculator’s answer in your head during planning meetings.

Frequently Asked Questions

It is the prefix length: the first 24 bits identify the network and the last 8 bits identify hosts. A /24 equals the mask 255.255.255.0 and holds 254 usable hosts.
In a normal subnet the first address names the network and the last one is the broadcast address, so neither can be given to a host. The exceptions are /31 and /32.
It is the inverse of the subnet mask. Access lists on routers use it, for example 0.0.0.255 for a /24.
Enter a larger prefix in the split box. A /24 split into /26 gives four subnets of 62 usable hosts each.

Disclaimer: results come from live requests and can change as records and servers change. We cannot guarantee that every answer matches what every visitor sees.