Subnet Calculator

Related

Use Cases

• When planning office or IDC room networks, calculate usable host counts and address ranges for different CIDR prefixes before finalizing VLAN and subnet plans
• When configuring router or switch interfaces, verify subnet mask and network address correctness to avoid outages from mask misconfiguration
• When writing firewall rules or ACLs, use wildcard mask results to quickly configure access control lists on Cisco/H3C devices
• When troubleshooting IP conflicts or connectivity issues, confirm network address and broadcast address boundaries are correct for given IP and mask
• When studying for CCNA/HCIA certifications, use this tool to verify manual calculations and deepen understanding of CIDR, VLSM and subnetting principles

Features

• One-click CIDR calculation: Enter IPv4 address + CIDR prefix (e.g. 192.168.1.0/24) to auto-calculate all subnet parameters, no lookup tables needed
• Eight complete results: Network address, broadcast address, subnet mask, wildcard mask, first host, last host, block size, binary mask
• Pure frontend local calculation: All computation done locally in browser, no data sent to server, instant input response with no latency
• One-click JSON copy: Results can be copied as JSON format for pasting into network design docs, scripts or reports
• Real-time input response: Calculates as you type, supports CIDR prefixes /8 to /32 full range; auto-hides results on format errors

How to Use

1. Enter a CIDR-format IPv4 address in the input box (e.g. 192.168.0.1/24); default example is 192.168.0.1/24
2. Tool calculates automatically in real-time; no button click needed. Results appear immediately when input format is correct
3. View 8 calculation results: Network, Broadcast, Netmask, Wildcard, First Host, Last Host, Block Size, Binary Mask
4. Click "Copy JSON Result" to copy complete calculation results to clipboard for network documentation or configuration records

FAQ

术语表

CIDR (Classless Inter-Domain Routing)

Notation using /prefix length to indicate the number of network bits, e.g. /24 means first 24 bits are network bits, replacing traditional Class A/B/C fixed divisions.

Subnet Mask

A 32-bit binary number where network bits are 1 and host bits are 0; bitwise AND with IP address yields the network address. E.g. /24 corresponds to 255.255.255.0.

Network Address

The first address of a subnet, with host bits all 0; used to identify the subnet itself and cannot be assigned to hosts.

Broadcast Address

The last address of a subnet, with host bits all 1; used to send broadcast packets to all hosts in the subnet and cannot be assigned to hosts.

Wildcard Mask

Result of bitwise inversion of subnet mask; 0 means must match, 1 means ignore; used in Cisco ACL and routing protocol configuration, e.g. 0.0.0.255.

VLSM (Variable Length Subnet Mask)

Using different length subnet masks within the same network to subnet on demand, improving IP address utilization.

Block Size

Total number of IP addresses in a subnet (including network and broadcast addresses), calculated as 2^(32-prefix length). E.g. /24 block size is 256.

Usable Hosts

Number of IPs assignable to end devices, equal to block size - 2 (subtracting network and broadcast addresses). /31 and /32 are special cases.

Prefix Length

The number after / in CIDR notation, indicating the number of network bits. Ranges from /0 (entire Internet) to /32 (single host).

Default Gateway

The IP address of the router interface in the subnet, usually the first usable host address, used to forward packets for cross-subnet communication.

CIDR Prefix to Subnet Mask / Usable Hosts Quick Reference

Private IP Address Ranges and Default Subnetting

Authoritative References

• Reference
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