Subnet Calculator
Works out the network address, broadcast address, usable host range and host count for any IPv4 address written in CIDR or netmask form.
About this tool
Subnet Calculator
Works out the network address, broadcast address, usable host range and host count for any IPv4 address written in CIDR or netmask form.
A subnet calculator turns an IPv4 address and a prefix length into the four numbers a network actually runs on: the network address, the broadcast address, the first and last usable host, and how many hosts fit. It applies the CIDR rules of RFC 4632 — network = address AND mask, broadcast = network OR the wildcard — plus the RFC 3021 exception that makes a /31 point-to-point link carry two usable addresses instead of none. It is for network engineers sizing a VLAN, cloud users choosing a VPC CIDR, and anyone studying for CCNA who wants to check their working.
How to use Subnet Calculator
- 1
Open Subnet Calculator on AltFTool — it loads instantly in your browser.
- 2
Type an address into IPv4 address (CIDR allowed) — the placeholder shows both accepted forms, 192.168.1.10 or 10.0.0.5/22 — and pick a Prefix length from the list beside it.
- 3
The Network heading updates with the CIDR and its usable host count, and the table beneath prints Address, Netmask and Network in binary so you can see where the prefix boundary falls.
- 4
Read Netmask, Wildcard mask, Network address, Broadcast address, First usable host, Last usable host, Host bits, Scope and Classful, use Split into subnets with a New prefix length, then press Copy result or Reset.
Why use Subnet Calculator
Handles the /31 and /32 edge cases
It applies the RFC 3021 point-to-point rule and the single-host route rule instead of reporting a nonsensical zero or negative host count.
Shows the binary
The address, netmask and network address are printed in binary so you can see exactly where the prefix boundary falls.
Names the block
RFC 1918 private space, RFC 6598 carrier NAT, RFC 3927 link-local and the RFC 5737 documentation ranges are identified by name and reference.
Common use cases
- Check whether a proposed VPC subnet is large enough: a /24 gives 254 usable hosts, a /26 gives 62, and a /28 gives only 14.
- Split an allocated 192.168.1.0/24 into four /26 blocks and read off each block's network, broadcast and host range before writing the DHCP scopes.
- Confirm that a router-to-router link configured as a /31 is legal and gives both ends a usable address under RFC 3021.
Frequently asked questions
How many usable hosts are in a /24?
254. A /24 leaves 8 host bits, so it spans 2^8 = 256 addresses, and two of those are not assignable: the all-zeros address is the network address and the all-ones address is the directed broadcast. The same subtraction gives 126 for a /25, 62 for a /26, 30 for a /27 and 14 for a /28.
Why does a /31 show 2 usable hosts instead of 0?
Because RFC 3021 removed the network and broadcast reservation for point-to-point links. A /31 has only two addresses, and subtracting two would leave none, so the standard says both are assignable — one per end of the link. That is why /31s are the normal choice for router-to-router links and save two addresses each compared with a /30.
What is a wildcard mask and how is it different from a netmask?
A wildcard mask is the bitwise inverse of the netmask: for a /24 the netmask is 255.255.255.0 and the wildcard is 0.0.0.255. Cisco ACLs and OSPF network statements take the wildcard form, where a 0 bit means "must match" and a 1 bit means "ignore", which is the opposite of a netmask.
Which IPv4 ranges are private?
Three, defined in RFC 1918: 10.0.0.0/8 (16,777,216 addresses), 172.16.0.0/12 (1,048,576 addresses) and 192.168.0.0/16 (65,536 addresses). Two more ranges are often mistaken for private: 100.64.0.0/10 is carrier-grade NAT space from RFC 6598, and 169.254.0.0/16 is link-local from RFC 3927, assigned automatically when DHCP fails.
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