IPv4 Subnetting Calculator
Use this tool to calculate subnet details from any IPv4 address and CIDR prefix length.
Tip: Press Enter in any field to calculate.
Why a subnetting calculator matters
Subnetting is one of those networking skills that seems difficult at first, but becomes straightforward with the right structure. A subnetting calculator helps you quickly identify the network address, broadcast address, host range, and total capacity for a given IPv4 block. Instead of doing binary math under time pressure, you can validate your design in seconds and avoid misconfigured routers, DHCP scopes, and ACLs.
Whether you are preparing for CCNA, building a home lab, or designing enterprise VLANs, a reliable calculator keeps your IP planning accurate. It is especially useful when you need to carve one large block into multiple smaller subnets using CIDR and variable-length subnet masks (VLSM).
What this calculator gives you
- Network address (first address in the subnet block)
- Broadcast address (last address in the subnet block)
- Usable host range (assignable addresses)
- Subnet mask and wildcard mask
- Total addresses and usable hosts
- IP class and address scope (private/public, multicast, loopback, etc.)
- Binary representation for troubleshooting and learning
Subnetting fundamentals in plain English
1) CIDR prefix
CIDR notation (for example, /24) tells you how many bits belong to the network portion of the address.
The remaining bits are host bits. More network bits means smaller subnets; fewer network bits means larger subnets.
2) Subnet mask
The prefix translates into a dotted decimal subnet mask. Example: /24 is 255.255.255.0.
A mask tells devices which bits identify the network and which bits identify individual hosts.
3) Usable hosts
Most subnets reserve two addresses: the network ID and broadcast. So usable hosts are typically:
2^(host bits) - 2. Special cases: /31 is commonly used for point-to-point links, and /32 represents a single host route.
CIDR quick reference
- /30: 4 total, 2 usable
- /29: 8 total, 6 usable
- /28: 16 total, 14 usable
- /27: 32 total, 30 usable
- /26: 64 total, 62 usable
- /25: 128 total, 126 usable
- /24: 256 total, 254 usable
- /23: 512 total, 510 usable
Example workflow: planning VLAN subnets
Suppose your branch office needs separate VLANs for staff, voice, and IoT devices. You estimate:
- Staff: 70 hosts
- Voice: 30 hosts
- IoT: 20 hosts
You can use host requirements to choose prefixes:
- 70 hosts →
/25(126 usable) - 30 hosts →
/27(30 usable) - 20 hosts →
/27(30 usable)
Then assign contiguous ranges from your parent block, document default gateways, and reserve room for growth. This approach prevents overlap and keeps route summarization clean.
Common subnetting mistakes to avoid
- Using a host IP as if it were the network address in router statements
- Forgetting that default usable host math changes for
/31and/32 - Overlooking growth and selecting subnets that are too small
- Mixing decimal and binary logic without a consistent method
- Not verifying private vs public addressing boundaries
Private IPv4 ranges to remember
10.0.0.0/8172.16.0.0/12192.168.0.0/16
These ranges are not routable on the public internet and are ideal for LAN design with NAT at the edge.
Final thoughts
A subnetting calculator is more than a convenience tool—it is a validation tool. Use it to verify each network block before deployment, double-check ACL boundaries, and reduce troubleshooting time later. The more you compare calculator output with manual subnetting steps, the faster your networking intuition improves.