Subnet Calculator
IPv4 CIDR calculator with usable ranges, wildcard masks, and VLSM split planning.
IPv4 calculator for planning. Point-to-point /31 and host /32 follow modern conventions. Always verify with your network standards.
Free IPv4 subnet calculator (CIDR)
Use this subnet calculator to plan IPv4 networks without spreadsheet math. Enter an IP address and CIDR prefix to get the network address, broadcast address, first and last usable hosts, subnet mask, wildcard mask, total addresses, and usable host count. Optional fields help with VLSM-style planning when you know how many hosts or subnets you need.
How CIDR subnet math works
IPv4 addresses are 32-bit numbers. A prefix length of n means the first n bits identify the network and the remaining 32 − n bits identify hosts.
| Prefix | Typical use | Usable hosts (classic) |
|---|---|---|
| /30 | Point-to-point (legacy) | 2 |
| /29 | Tiny segment | 6 |
| /28 | Small office VLAN | 14 |
| /27 | Small department | 30 |
| /26 | Medium LAN | 62 |
| /24 | Common LAN default | 254 |
| /16 | Large private block | 65,534 |
Modern point-to-point links often use /31 (RFC 3021) with 2 addresses and no classic broadcast. Host routes use /32.
Formulas this tool uses
- Subnet mask from prefix: binary ones for network bits, zeros for host bits
- Network address = IP AND mask
- Broadcast = network OR wildcard
- Usable range = network+1 through broadcast−1 (except /31 and /32 special cases)
- Usable hosts = 2^(32−prefix) − 2 when prefix ≤ 30
These are the same fundamentals taught in CCNA-style networking and used by popular free subnet calculators from SolarWinds, MXToolbox, and open-source ipcalc tools.
VLSM and real network design
Variable Length Subnet Masking (VLSM) lets you carve a parent block into unequal children so a 12-host printer VLAN does not waste a full /24. Use the optional “hosts needed” and “subnets needed” fields for a first-pass split, then document the plan in IPAM.
Good production design also includes:
- Separate user, server, voice, IoT, and guest networks
- Firewall zones between trust levels
- DHCP scopes that match usable ranges
- Documentation for reverse DNS and ACLs
Common mistakes this calculator helps catch
- Using /24 everywhere and running out of private space later
- Forgetting network and broadcast reservations when ordering gear
- Mismatched masks on two ends of a link
- Overlapping VPN and LAN ranges after an acquisition
- Planning IoT cameras on the same flat LAN as domain controllers
Networking services from Secure Techies
If you are redesigning VLANs, deploying SD-WAN, or hardening edge firewalls, pair this tool with network security services and managed infrastructure. For camera networks, also size storage with the surveillance storage calculator.
Ready for a network review? Contact Secure Techies for a free assessment.
Frequently asked questions
What is a subnet calculator?
A subnet calculator converts an IP address and CIDR prefix (or mask) into network details: network address, broadcast address, usable host range, subnet mask, wildcard mask, and host counts.
What is CIDR notation?
CIDR writes a network as address/prefix, for example 192.168.10.0/24. The prefix is the number of network bits (0–32 for IPv4). Larger prefixes mean smaller subnets.
How many usable hosts are in a /24?
A /24 has 256 total addresses and typically 254 usable hosts (network and broadcast reserved). /31 and /32 are special cases used for point-to-point links and single hosts.
What is a wildcard mask?
A wildcard mask is the inverse of the subnet mask, commonly used in Cisco ACL and OSPF configurations. For 255.255.255.0 the wildcard is 0.0.0.255.
Can this tool do VLSM planning?
Yes. Enter hosts needed or subnets needed to see a recommended prefix and a sample split table. For complex multi-site designs, validate against your IPAM standards.
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