Network

Subnet Calculator

Calculate network, broadcast, host range, and mask details from CIDR.

Formula reviewed: 2026-02-14 Network

Subnet Calculator derives the network address, broadcast address, usable host range, and subnet mask from CIDR input. CIDR notation combines an IP address with a prefix length, such as /24, where the prefix marks how many leading bits identify the network rather than individual hosts. The network address is the first address in the block, the broadcast address is the all-hosts address in IPv4, and the usable range excludes addresses reserved for network and broadcast roles. Subnet masks express the same boundary in dotted decimal form. This tool is useful for network planning and troubleshooting because it makes address boundaries explicit, but production changes should still be checked against router, cloud-provider, DHCP, and firewall configuration.

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Input Pattern

Enter values in the left panel, keep units explicit, run the calculation, then copy or share the result. Invalid fields are highlighted immediately.

How to use this tool

Enter CIDR for the subnet calculator, keeping units, dates, or text format consistent with the form labels.
Confirm address formats, masks, ports, or hostnames match the network environment you are checking.
Click "Run the tool" and review Subnet Input, Result for the primary output.
Compare the output with device, provider, or DNS authority settings before applying a live network change.

Subnet Input

Result

Network: 192.168.10.0/24

Subnet mask: 255.255.255.0

Wildcard: 0.0.0.255

Broadcast: 192.168.10.255

First host: 192.168.10.1

Last host: 192.168.10.254

Usable hosts: 254

IPv4 Subnetting Fundamentals

Networks, Hosts, and Prefix Lengths

IPv4 subnetting divides address space into smaller networks. An IPv4 address contains 32 bits. A prefix length, such as /24 or /27, says how many of those bits identify the network portion. The remaining bits identify host addresses within that network.

A /24 leaves 8 host bits, which gives 256 total addresses. A /27 leaves 5 host bits, which gives 32 total addresses. In traditional subnet planning, the first address is the network address and the last is the broadcast address, leaving fewer usable host addresses. Understanding this bit split is the foundation for reading routing tables, firewall rules, and allocation plans.

Masks as Bit Boundaries

A subnet mask is the dotted-decimal form of the prefix boundary. For example, /24 corresponds to 255.255.255.0 because the first 24 bits are set to one. /26 corresponds to 255.255.255.192 because the first two bits of the last octet are part of the network.

The mask is not arbitrary decoration; it controls how devices decide whether a destination is local or must be sent to a router. Two devices with compatible addresses but mismatched masks may disagree about what is local, causing confusing connectivity failures. Precise mask handling is therefore a practical operations skill, not just binary arithmetic.

Planning Address Blocks

Good subnet planning balances current host counts, future growth, route simplicity, and security boundaries. Very large subnets waste addresses and enlarge broadcast domains. Very small subnets conserve addresses but can become brittle when teams add devices or services.

Network operators often allocate by function: user devices, servers, management interfaces, point-to-point links, lab systems, and guest networks may each receive distinct ranges. That separation makes firewall policy easier and limits the blast radius of mistakes. Address plans also need summarization in mind, because contiguous blocks can often be represented by shorter aggregate routes.

Common Mistakes

Subnetting mistakes often come from off-by-one thinking. The network and broadcast addresses are not ordinary host addresses in conventional IPv4 subnets. Another common error is mixing CIDR notation and dotted masks without checking that they match. A third is assuming that adjacent-looking addresses belong to the same subnet when the prefix boundary says otherwise.

Modern cloud networks, point-to-point links, and special cases may relax or reinterpret some older conventions, but the core arithmetic still matters. When connectivity looks mysterious, confirming the address, prefix length, gateway, and route selection is often the fastest path to clarity.

Formula or method

The calculator parses CIDR notation into an IP address and prefix length.
The prefix length determines how many address bits belong to the network portion.
For conventional IPv4 subnets, the first address is the network address and the last address is the broadcast address.
Usable host count excludes reserved network and broadcast addresses except in special cases handled by the network design.

Worked example

Checking a /27 allocation

CIDR: 192.0.2.64/27

Result: The block contains 32 total addresses, with a network address of 192.0.2.64 and a broadcast address of 192.0.2.95.

In a conventional IPv4 LAN, usable host addresses run from 192.0.2.65 through 192.0.2.94. Confirm gateway, DHCP, and firewall assumptions separately.

How to interpret the result

Subnet results define address boundaries and usable ranges for the entered CIDR. Confirm the result against the actual network plan before changing routing, firewall, or cloud settings.

The network and broadcast rows define the block boundary; they are not ordinary host addresses in traditional IPv4 subnets.
Usable host count helps capacity planning, but reservations for gateways, VIPs, DHCP pools, and infrastructure reduce practical capacity.
A matching subnet mask must be configured consistently on hosts, routers, firewall rules, and cloud network objects.
Cloud and point-to-point networks can reserve or reinterpret addresses differently, so provider documentation still matters.

Common mistakes

Assuming adjacent-looking IP addresses belong to the same subnet without checking the prefix.
Mixing CIDR prefix length and dotted subnet mask values that do not match.
Forgetting that cloud providers may reserve additional addresses inside a subnet.
Applying LAN broadcast assumptions to every network type without checking context.

Formula References

IPv4/IPv6 addressing rules and common protocol arithmetic.
DNS/WHOIS/network calls depend on external resolvers and current network state.

Assumptions

CIDR notation is valid and represents the intended IPv4 network.
Usable-host interpretation follows common IPv4 subnet conventions.

Review note and limitations

Method - standard IPv4 CIDR subnet arithmetic.

Does not inspect live devices, DHCP scopes, cloud reservations, NAT, route tables, or overlapping allocations.
IPv6 planning may use different conventions around host ranges and subnet sizing.

Planning calculation only. Verify live network changes against source-of-truth IPAM, cloud configuration, routers, and firewalls.

FAQ

What does CIDR notation mean?

CIDR combines an IP address with a prefix length, such as 192.0.2.10/24. The prefix length says how many leading bits identify the network.

Why are network and broadcast addresses shown separately?

They define the start and end of a conventional IPv4 subnet and are usually not assigned to ordinary hosts.

Can I use this for cloud subnet planning?

Yes for the address math, but cloud providers often reserve platform addresses and may apply provider-specific routing rules.

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