Subnet Calculator

About this tool

Subnet Calculator on UtilityKit resolves an IPv4 address and prefix length into its network parameters in your browser. Type an address such as 192.168.1.42 in the IPv4 address field, set the CIDR prefix (0-32), click Calculate, and receive the subnet in CIDR form, the netmask in dotted-decimal, the wildcard mask, the broadcast address, the first and last usable host, and the usable host count, all computed with no server required. If you enter a host address, the Subnet line shows the canonical network, so 192.168.1.50 with prefix 24 becomes 192.168.1.0/24. /31 point-to-point and /32 host routes are handled. The calculator works on one block at a time, which suits checking firewall rules, cloud security group ranges, VPC subnet sizes or CCNA practice answers without manual binary arithmetic and the off-by-one errors it invites.

Why use it

• Eliminates Binary Arithmetic: Converting CIDR to subnet mask and host ranges by hand requires error-prone binary AND operations. The calculator handles all bit arithmetic instantly and accurately.

• Cloud & Firewall Ready: AWS VPC, Azure VNET, GCP VPC, and most firewall UIs accept CIDR input directly. Verify your address ranges before entering them to avoid over-permissive or broken rules.

• Host-Address Correction: Enter any address inside a subnet and the tool reports the true network boundary, catching rules written with a host address where a network address belongs.

• VLSM Planning: Change the prefix length and recalculate to size subnets for different departments or availability zones without maintaining a spreadsheet.

• Exam & Study Aid: CCNA, CompTIA Network+, and similar certifications test subnet calculations. This tool lets you check your manual work instantly during practice.

• Fully Browser-Side: Internal IP addresses and network topologies never leave your device — all computation runs locally with no server round-trip.

How to use

1. Type an IPv4 address in dotted-decimal into the IPv4 address field, for example 10.0.0.0 or 192.168.100.64, and the prefix length (0-32) into the CIDR field.
2. Click Calculate. If you entered a host address instead of the network address (e.g. 192.168.1.50 with CIDR 24), the Subnet line shows the canonical network 192.168.1.0/24.
3. Read the results: Subnet, Netmask, Wildcard, Broadcast, First host, Last host, and Usable hosts.
4. Note the usable host formula: 2^(32−prefix) − 2, subtracting the network and broadcast addresses. A /30 gives 2 hosts; a /24 gives 254.
5. For VLSM planning, change the CIDR value and click Calculate again to see how host count and address range change — going from /24 to /25 halves the usable hosts.
6. Select the result text and copy it into documentation or a network diagram tool.

When it helps

• When designing a VPC or VNET and you need to confirm that each planned CIDR block provides enough host addresses for its tier.
• When writing firewall or security group rules and you need the exact network address and mask to avoid accidentally allowing too broad a range.
• When troubleshooting a routing issue and you need to verify whether a given IP falls within a particular subnet's host range.
• When documenting a network topology and you need broadcast addresses, wildcard masks, or host counts for every segment.
• When studying for CCNA, Network+, or similar networking certifications and you want to verify your manual subnet calculations.
• When migrating an on-premise network to a cloud provider and you need the dotted netmask that matches a given CIDR prefix length.

Examples

IPv4 address 192.168.1.50, CIDR 24

Subnet: 192.168.1.0/24 | Netmask: 255.255.255.0 | Wildcard: 0.0.0.255 | Broadcast: 192.168.1.255 | First host: 192.168.1.1 | Last host: 192.168.1.254 | Usable hosts: 254

IPv4 address 10.0.0.0, CIDR 30

Subnet: 10.0.0.0/30 | Netmask: 255.255.255.252 | Wildcard: 0.0.0.3 | Broadcast: 10.0.0.3 | First host: 10.0.0.1 | Last host: 10.0.0.2 | Usable hosts: 2

IPv4 address 172.31.0.0, CIDR 16

Subnet: 172.31.0.0/16 | Netmask: 255.255.0.0 | Wildcard: 0.0.255.255 | Broadcast: 172.31.255.255 | First host: 172.31.0.1 | Last host: 172.31.255.254 | Usable hosts: 65,534

Tips

• For point-to-point WAN links between two routers, use a /30 (2 usable hosts) to conserve address space instead of wasting a /24 on a two-device link.
• When planning a three-tier VPC (web, app, database), allocate larger subnets (/23 or /22) to tiers that auto-scale and smaller ones (/27 or /28) to tightly controlled database tiers.
• The subnet mask in dotted-decimal maps directly to CIDR: 255.255.255.0 = /24, 255.255.255.128 = /25, 255.255.255.192 = /26, 255.255.255.224 = /27.
• Always leave at least one /16 block unallocated when designing a large-organisation address plan — growth inevitably requires more space than initially estimated.
• Cross-check your output against RFC 1918 private ranges: if your subnet contains 10.x.x.x, 172.16-31.x.x, or 192.168.x.x addresses, it is non-routable on the public internet and appropriate for internal use only.

Frequently Asked Questions

Glossary

CIDR

Classless Inter-Domain Routing — a method for allocating IP addresses using a variable-length prefix instead of the fixed class system (Class A/B/C). Written as address/prefix-length, e.g. 10.0.0.0/8.

Network Address

The first address in a subnet, where all host bits are zero. Identifies the subnet itself and cannot be assigned to any host device.

Broadcast Address

The last address in a subnet, where all host bits are one. Packets sent to this address are delivered to all hosts in the subnet. It cannot be assigned to a host.

Subnet Mask

A 32-bit number that divides an IP address into network and host portions. Written in dotted-decimal (e.g. 255.255.255.0) or as a prefix length after a slash (e.g. /24).

Wildcard Mask

The bitwise inverse of the subnet mask. Used in Cisco ACL and OSPF configurations to specify which bits of an address are significant. A /24 subnet mask of 255.255.255.0 has wildcard mask 0.0.0.255.

VLSM

Variable Length Subnet Masking — the practice of subdividing an address space into subnets of different sizes by using different prefix lengths, allowing efficient allocation of IP addresses across networks with varying host counts.