Subnet Calculator

About IPv4 Subnet Calculator

Our free IPv4 subnet calculator performs bit-accurate subnet calculations for any IP address and CIDR prefix length. Enter an IP address, select a subnet mask using the interactive CIDR slider or type a dotted-decimal mask, and instantly see the network address, broadcast address, usable host range, wildcard mask, IP class, and binary representations. This is an essential tool for network engineers, system administrators, and anyone working with IP addressing and subnetting.

The calculator uses exact 32-bit binary arithmetic (AND, OR, and NOT on the address and mask), so results are correct for every CIDR prefix from /0 to /32. Network address (IP AND mask), broadcast address (network OR NOT mask), and host counts all update instantly as you type.

Use it to split an office /22 into VLAN subnets, write firewall rules for an address range, drill for the CCNA or CompTIA Network+ certification, or check how many usable hosts fit in a /27 (30). Answers come with binary breakdowns and quick-select buttons for common subnet sizes.

Key Features

• Bit-Accurate Calculations: Exact unsigned 32-bit arithmetic on every IP and mask value, so addresses from 128.0.0.0 upward never produce negative or wrong results
• CIDR Slider: Interactive range slider from /0 to /32 that instantly updates the dotted-decimal subnet mask and all calculations
• Dotted Notation Input: Enter subnet masks in dotted-decimal format (e.g. 255.255.255.0) with automatic CIDR conversion and mask validation
• Quick-Select Buttons: One-click buttons for the most commonly used CIDR prefixes: /8, /16, /24, /25, /26, /27, and /28
• Complete Subnet Details: Network address, broadcast address, first and last usable hosts, total hosts, usable hosts, subnet mask, wildcard mask, and CIDR notation
• IP Class Detection: Automatically identifies the classful IP address class (A, B, C, D, or E) with corresponding address ranges
• Private/Public Detection: Determines whether the IP address falls within RFC 1918 private address space (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) or is publicly routable
• Binary Representations: Full binary breakdowns of IP address, subnet mask, network address, and broadcast address for educational and debugging purposes
• Edge Case Handling: Correct behavior for /31 (point-to-point links per RFC 3021) and /32 (single host) subnets
• Input Validation: Real-time validation of IP address format and subnet mask contiguity with clear error messages

How Subnetting Works

Subnetting is the process of dividing a larger IP network into smaller, more manageable sub-networks (subnets). Every IPv4 address is a 32-bit number divided into a network portion and a host portion. The subnet mask defines where this division occurs: bits set to 1 represent the network portion, and bits set to 0 represent the host portion.

1. Network Address: Obtained by performing a bitwise AND between the IP address and subnet mask. This is the identifier for the subnet itself and cannot be assigned to a host.
2. Broadcast Address: Obtained by performing a bitwise OR between the network address and the inverted subnet mask (wildcard mask). Packets sent to this address reach all hosts on the subnet.
3. Usable Host Range: All addresses between network + 1 (first usable) and broadcast - 1 (last usable). For a /24 subnet this gives 254 usable addresses.
4. Wildcard Mask: The bitwise NOT of the subnet mask. Used in access control lists (ACLs) on routers to specify which bits of an address should be matched.
5. Host Count: For a prefix length of N, the total number of addresses is 2^(32-N). Usable hosts are total minus 2 (network and broadcast), except for /31 and /32 special cases.

Understanding CIDR Notation

CIDR (Classless Inter-Domain Routing) notation represents an IP address and its associated network prefix length in the format address/prefix. For example, 192.168.1.0/24 means the first 24 bits are the network portion, leaving 8 bits for host addresses (256 total addresses, 254 usable). CIDR replaced the older classful addressing system (Class A, B, C) to allow more flexible allocation of IP address space, reducing waste and slowing the exhaustion of the IPv4 address pool.

The prefix length directly corresponds to the number of contiguous 1-bits in the subnet mask. A /24 prefix equals a mask of 255.255.255.0 (24 ones followed by 8 zeros in binary). A /16 prefix equals 255.255.0.0, and a /8 prefix equals 255.0.0.0. Shorter prefixes create larger networks with more hosts; longer prefixes create smaller, more targeted subnets.

IP Address Classes

Although CIDR has largely replaced classful addressing, understanding IP classes remains important for certification exams, legacy network design, and default subnet mask assignments:

• Class A (0.0.0.0 - 127.255.255.255): First bit is 0. Default mask /8. Designed for very large networks with up to 16.7 million hosts per network. Private range: 10.0.0.0/8.
• Class B (128.0.0.0 - 191.255.255.255): First two bits are 10. Default mask /16. Supports up to 65,534 hosts per network. Private range: 172.16.0.0/12.
• Class C (192.0.0.0 - 223.255.255.255): First three bits are 110. Default mask /24. Supports up to 254 hosts per network. Private range: 192.168.0.0/16.
• Class D (224.0.0.0 - 239.255.255.255): First four bits are 1110. Reserved for multicast groups. Not used for standard host addressing.
• Class E (240.0.0.0 - 255.255.255.255): First four bits are 1111. Reserved for experimental and future use.

Use Cases

• Network Design: Plan IP address allocation for office buildings, data centers, and campus networks by determining the right subnet size for each VLAN or department.
• Firewall Configuration: Calculate network and wildcard masks needed for access control lists (ACLs) and firewall rules to permit or deny traffic to specific subnets.
• CCNA / CompTIA Study: Practice subnetting calculations for networking certification exams with instant binary verification of your manual work.
• Troubleshooting: Verify that two hosts are on the same subnet, check broadcast addresses, and confirm that routing table entries are correctly configured.
• Cloud Infrastructure: Design VPC subnets in AWS, Azure, or GCP by determining the right CIDR block sizes for public subnets, private subnets, and database tiers.
• Home Networking: Understand your home router's subnet configuration, determine how many devices can connect, and plan static IP assignments.
• Security Auditing: Identify whether IP addresses fall within private (RFC 1918) ranges or are publicly routable when reviewing network logs and security alerts.
• Documentation: Generate accurate subnet details for network documentation, IP address management (IPAM) records, and change management tickets.

Frequently Asked Questions

Tips & Best Practices

• Right-size your subnets: Choose the smallest subnet that accommodates your current host count plus 20-30% growth. Over-allocating wastes address space; under-allocating forces painful re-addressing.
• Use the binary view for learning: Examining the binary representations of IP addresses and masks builds strong subnetting intuition and helps you solve problems faster on certification exams.
• Plan for VLANs: Align each VLAN with its own subnet. A common pattern is /24 for user VLANs, /28 or /29 for management networks, and /30 or /31 for router point-to-point links.
• Document everything: Record the network address, CIDR notation, usable range, gateway (typically first or last usable host), and purpose for every subnet in your IP address management system.
• Reserve addresses consistently: Adopt a standard convention such as using the first usable address as the default gateway and the last few addresses for infrastructure devices (DNS, DHCP servers).
• Use private addresses internally: Use RFC 1918 private address space for internal networks and NAT for internet access. This conserves public IP addresses and adds a layer of security.