IPv4 Subnet Calculator
Enter an IP address and CIDR prefix to instantly calculate network details.
What jodies subnet calculator helps you do
Subnetting can feel intimidating when you are trying to plan VLANs, prepare firewall rules, or troubleshoot why hosts cannot reach each other. jodies subnet calculator makes this practical: you provide an IPv4 address and prefix, and the tool calculates everything you usually need in one pass—network address, broadcast, usable host range, wildcard mask, and host counts.
This is useful whether you are a student studying for Network+, CCNA, or security certifications, or an engineer building production network segments for offices, cloud environments, and labs.
How to use this subnet calculator
Step 1: Enter an IPv4 address
Type any valid IPv4 address such as 192.168.10.42. The address can be a host IP inside the subnet; it does not need to be the first address in the block.
Step 2: Enter CIDR prefix (or subnet mask)
Use the prefix field (for example /24, /27, or /30). You can also type a subnet mask directly, and the calculator will infer the prefix as long as the mask is valid and contiguous.
Step 3: Read the results
- Network Address: The first address in the subnet block.
- Broadcast Address: The last address in the subnet block.
- First/Last Host: Typical assignable host range.
- Total and Usable Addresses: Capacity planning values.
- Wildcard Mask: Handy for ACLs and route filters.
Practical subnetting examples
Small office VLAN
If your user VLAN needs around 25 devices, a /27 gives 32 total addresses and 30 usable. That keeps the segment tight and easier to secure versus dropping everyone into a large /24.
Point-to-point links
For routed links between two devices, many teams use /31 or /30. /31 is efficient and supported by modern routing stacks, while /30 remains common in legacy environments.
Lab segmentation
In homelabs, splitting one /24 into several /28 or /29 networks is a great way to isolate test systems, management interfaces, and IoT devices without buying additional hardware.
CIDR quick reference
- /24 → 255.255.255.0 → 256 total / 254 usable
- /25 → 255.255.255.128 → 128 total / 126 usable
- /26 → 255.255.255.192 → 64 total / 62 usable
- /27 → 255.255.255.224 → 32 total / 30 usable
- /28 → 255.255.255.240 → 16 total / 14 usable
- /29 → 255.255.255.248 → 8 total / 6 usable
- /30 → 255.255.255.252 → 4 total / 2 usable
- /31 → 255.255.255.254 → 2 total / 2 usable (point-to-point)
- /32 → 255.255.255.255 → 1 total / 1 usable (single host route)
Common mistakes this tool helps prevent
- Using non-contiguous subnet masks like 255.0.255.0 (invalid for standard CIDR subnetting).
- Forgetting that network and broadcast addresses are usually not assignable in traditional subnets.
- Sizing a subnet too small, then running out of addresses during growth.
- Using a large flat subnet when smaller segmented networks improve performance and security.
Final note
Subnetting is one of those skills that compounds over time. The more you model real networks, the faster it becomes. Keep this calculator handy for daily design and troubleshooting, and pair it with clear IPAM documentation so your network stays predictable as it scales.