IPv4 Subnet Calculator
Quickly calculate network details from an IP and subnet. You can enter subnet as CIDR (/24 or 24) or dotted mask (255.255.255.0).
What is IP subnetting?
IP subnetting is the process of splitting a larger network into smaller, manageable networks called subnets. In IPv4, this is done by using a subnet mask or CIDR prefix (for example, /24, /27, or /30) to define which bits represent the network and which bits represent hosts.
Subnetting helps reduce broadcast traffic, improve security boundaries, simplify routing, and make better use of available address space.
Why an IP subnet calculator is useful
- Speed: Instantly compute network address, broadcast, and host range.
- Accuracy: Avoid manual binary mistakes during planning.
- Troubleshooting: Quickly verify if two hosts are in the same subnet.
- Documentation: Generate clean subnet details for network diagrams and change tickets.
Core subnetting terms
IP Address
A unique 32-bit identifier (IPv4) represented as four octets, such as 192.168.1.130.
Subnet Mask
A 32-bit value that marks network bits as 1 and host bits as 0, like 255.255.255.0.
CIDR Prefix
The number after the slash in CIDR notation (for example, /24) representing how many bits are network bits.
Network Address and Broadcast Address
- Network address: First address in the subnet (all host bits = 0).
- Broadcast address: Last address in the subnet (all host bits = 1).
How to read calculator output
When you enter an IP and subnet, you get:
- Network address (subnet ID)
- Broadcast address
- First and last usable host
- Total and usable addresses
- Wildcard mask (useful for ACLs)
- IP class and private/public scope
Common CIDR quick reference
| CIDR | Subnet Mask | Total Addresses | Typical Usable Hosts |
|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |
Subnetting best practices
- Plan with growth in mind: Leave room for future devices and VLAN expansion.
- Use consistent addressing: Keep subnet boundaries aligned by location, function, or security zone.
- Document everything: Include gateway, VLAN ID, DHCP scope, and purpose.
- Validate before deployment: Test gateway and host ranges to prevent overlap.
Common mistakes to avoid
- Using non-contiguous subnet masks.
- Assigning the network or broadcast address to a host (except special /31 behavior).
- Overlapping subnets across sites or VPN domains.
- Forgetting to update routing and firewall rules after subnet changes.
Final thoughts
An IP calculator for subnetting is one of the most practical tools for engineers, admins, students, and anyone building reliable networks. Use the calculator above to test scenarios quickly, then apply the results in your IP plan, DHCP scopes, ACLs, and routing tables.