IPv4 planning utility
Calculate a practical IPv4 subnet report from an address and CIDR prefix or dotted netmask. The calculator shows network ID, broadcast, usable range, mask, wildcard, binary boundaries, a same-subnet check, equal-size splits and a VLSM plan that can be copied into documentation.
This tool focuses on IPv4 subnet math. Prefix edge cases such as /31 point-to-point blocks and /32 host routes are handled explicitly.
What this IP subnet calculator is for
Subnet math is easy to get almost right and expensive to get slightly wrong. A host address can look familiar while belonging to a different block than the firewall rule you planned. A /24 can feel natural until a VLAN needs to be split into smaller pieces. A VPN design can work on paper and still overlap with an office network. This subnet calculator turns an IPv4 address and prefix into the values an administrator usually needs before making a routing, DHCP, ACL or documentation change.
The summary gives the network ID, broadcast address, first and last usable addresses, total address count, usable host capacity, subnet mask and wildcard mask. The deeper tabs show why those values are correct. The binary boundary makes network bits and host bits visible. The membership check confirms whether a second address belongs to the same block. The equal split planner and VLSM planner help when one parent network must serve several smaller networks.
How to read an IPv4 subnet report
- The network ID identifies the block and is normally not assigned to a host.
- The broadcast address is the last address of a classic IPv4 subnet.
- The usable range sits between network and broadcast for most prefixes.
- The subnet mask is the dotted form of the prefix, such as 255.255.255.0 for /24.
- The wildcard mask is the inverse mask used by some ACL and routing syntax.
- Host bits decide the size of the block; network bits decide which block it is.
Why /31 and /32 need a careful reading
Most IPv4 lessons teach network address, usable hosts and broadcast with a familiar /24 or /30 example. That model is useful, but it is not the whole story. A /31 is often used for point-to-point links where both addresses can be endpoints. A /32 represents one address, commonly in host routes, loopback documentation or allowlist rules. A good subnet calculator should make those edge cases explicit instead of silently subtracting two addresses from every prefix.
Subnet planning and VLSM in practice
An equal-size split is helpful when several VLANs need similar capacity. If a /24 is split into four equal blocks, each child is a /26. VLSM is better when needs are uneven. A user network might need 120 hosts while a management network needs only 12. Allocating the largest needs first reduces fragmentation and keeps the plan easier to review. Keep growth room in mind, reserve infrastructure addresses deliberately, and document the parent network that each allocation came from.
Common mistakes worth catching early
- Using a host IP where a network ID is expected in a firewall or route.
- Confusing the prefix with a dotted mask or a wildcard mask.
- Forgetting that a DHCP pool should avoid reserved gateway and infrastructure addresses.
- Designing private networks that overlap with a VPN, cloud VPC or remote office.
- Assuming a public address range behaves like an internal RFC1918 lab range.
Common questions
Can I enter a netmask instead of a CIDR prefix?
Yes. Enter a prefix such as /23 or a dotted mask such as 255.255.254.0. The tool validates that dotted masks are contiguous.
Does the compare field prove routing will work?
No. It proves whether two addresses fall inside the same calculated subnet. Routing, VLANs, firewalls and host masks still decide whether real traffic can pass.
Should I use VLSM for every design?
No. Use it when different subnets need different capacities. Equal blocks can be simpler when operational clarity matters more than saving a small number of private addresses.
What does a /24 subnet mean?
The /24 is the prefix length: 24 bits are the network and 8 bits are hosts, giving 256 addresses (254 usable). A /16 gives 65536, a /30 gives 4 (2 usable).
How many usable hosts are in a subnet?
Total addresses minus two, because the network address and the broadcast address are reserved. A /24 therefore has 254 usable hosts.
What is CIDR notation?
Classless Inter-Domain Routing notation writes an address with its prefix length, like 192.168.1.0/24. It replaced the old class A/B/C system and allows flexible subnet sizes.
