IP Subnet Calculator — Free Online IPv4 Subnet Tool

How to Use the IP Subnet Calculator

1. Enter an IP address: Type any valid IPv4 address in dotted decimal notation into the IP Address field. For example, 192.168.1.0, 10.0.0.1, or 172.16.50.100. The address does not need to be the network address; the calculator will determine the correct network address based on the CIDR prefix you provide. Each octet should be a number from 0 to 255, separated by periods.
2. Set the CIDR prefix length: Enter the CIDR prefix length as a number from 0 to 32. This value determines the size of the subnet. A /24 gives you 254 usable hosts, a /25 gives 126, a /26 gives 62, and so on. Each time you increase the CIDR by 1, the number of available hosts is halved. Common values for home networks are /24, for enterprise departments /22 to /26, and for point-to-point links /30 or /31.
3. Review subnet details: The results panel instantly displays all critical subnet information. The network address is the starting boundary of the subnet. The broadcast address is the ending boundary. The subnet mask shows which bits belong to the network versus host portions. The wildcard mask is the inverse of the subnet mask, used in Cisco ACLs and OSPF. First and last host addresses define the assignable range for devices.
4. Use the results for network planning: Apply the calculated subnet information to configure your routers, switches, firewalls, and DHCP scopes. The network address goes into routing tables. The usable host range defines your DHCP pool. The broadcast address should be excluded from static assignments. The wildcard mask is needed for Cisco ACL entries and OSPF area definitions.

All results update instantly as you change the IP address or CIDR prefix. Experiment with different prefix lengths to find the optimal subnet size for your network requirements.

Subnet Calculation Formula

Variables Explained

• IP Address: A 32-bit number uniquely identifying a device on a network, written as four octets in dotted decimal notation (e.g., 192.168.1.100). Each octet represents 8 bits and ranges from 0 to 255. The IP address contains both the network identifier and the host identifier.
• CIDR Prefix: A number from 0 to 32 indicating how many of the 32 bits are used for the network portion. A /24 means the first 24 bits are the network and the last 8 bits are for hosts. This replaced the older classful system where only /8, /16, and /24 were standard network boundaries.
• Subnet Mask: A 32-bit value where the network bits are all 1s and the host bits are all 0s. For /24, the mask is 11111111.11111111.11111111.00000000 or 255.255.255.0 in decimal. The AND operation between the IP and mask isolates the network portion.
• Wildcard Mask: The bitwise complement of the subnet mask. For a /24 subnet with mask 255.255.255.0, the wildcard mask is 0.0.0.255. Used in Cisco IOS configurations for ACLs and routing protocols where a 0 bit means "must match" and a 1 bit means "don't care."

Step-by-Step Example

Calculate the subnet details for the IP address 10.50.100.200 with a /20 prefix:

1. Convert the /20 prefix to a subnet mask: first 20 bits are 1s = 255.255.240.0
2. AND the IP with the mask: 10.50.100.200 AND 255.255.240.0 = 10.50.96.0 (network address)
3. Calculate wildcard mask: 255 - 255 = 0, 255 - 255 = 0, 255 - 240 = 15, 255 - 0 = 255 = 0.0.15.255
4. OR the network with the wildcard: 10.50.96.0 OR 0.0.15.255 = 10.50.111.255 (broadcast address)
5. First host: network + 1 = 10.50.96.1
6. Last host: broadcast - 1 = 10.50.111.254
7. Total hosts: 2^(32-20) = 2^12 = 4,096
8. Usable hosts: 4,096 - 2 = 4,094

This /20 subnet provides 4,094 usable host addresses ranging from 10.50.96.1 to 10.50.111.254. It covers 16 Class C equivalent blocks, making it suitable for a medium-sized campus or data center segment. The address 10.x.x.x falls in the Class A private range, confirming it is a non-routable internal address.

Practical Examples

Example 1: Nathan's Home Network Setup

Nathan is setting up a home network with 12 devices including laptops, phones, a smart TV, a gaming console, and IoT devices. He wants to configure his router's DHCP scope properly:

• IP address: 192.168.1.0
• CIDR prefix: /24
• Network address: 192.168.1.0
• Broadcast address: 192.168.1.255
• Usable hosts: 254 (192.168.1.1 to 192.168.1.254)

Nathan assigns 192.168.1.1 to his router (default gateway), reserves 192.168.1.2 through 192.168.1.10 for static devices like his NAS and printer, and sets the DHCP pool from 192.168.1.50 to 192.168.1.200. This gives him plenty of room for current devices and future additions while keeping static and dynamic address ranges separated.

Example 2: Lisa's Office Network Segmentation

Lisa is a network administrator designing a segmented network for a 200-person company with four departments. She uses the subnet calculator to plan her VLSM scheme starting with 10.10.0.0/16:

• Engineering (80 devices): 10.10.0.0/25 — 126 usable hosts (10.10.0.1 to 10.10.0.126)
• Sales (60 devices): 10.10.0.128/26 — 62 usable hosts (10.10.0.129 to 10.10.0.190)
• Marketing (40 devices): 10.10.1.0/26 — 62 usable hosts (10.10.1.1 to 10.10.1.62)
• Executive (15 devices): 10.10.1.64/27 — 30 usable hosts (10.10.1.65 to 10.10.1.94)

By using different subnet sizes for each department, Lisa avoids wasting IP addresses and creates natural firewall boundaries between departments. She configures inter-VLAN routing on the core switch and applies access control lists to restrict executive subnet access to authorized personnel only.

Example 3: Derek's Data Center Planning

Derek is designing the IP addressing scheme for a new data center that will host 500 virtual machines across three availability zones. He uses a /21 block:

• Parent block: 172.16.0.0/21
• Total addresses: 2,048
• Usable hosts: 2,046
• Zone A: 172.16.0.0/23 — 510 usable hosts
• Zone B: 172.16.2.0/23 — 510 usable hosts
• Zone C: 172.16.4.0/23 — 510 usable hosts

Each zone gets a /23 subnet with 510 usable addresses, providing ample room for the planned 167 VMs per zone plus future growth. Derek reserves the remaining 172.16.6.0/23 block for management networks, monitoring systems, and out-of-band access. The 172.16.x.x range is a Class B private address space, ideal for internal data center use.

CIDR Subnet Reference Table

Each CIDR decrease by 1 doubles the number of available hosts. A /31 has 2 hosts (point-to-point), a /32 has 1 (single host).

Tips and Complete Guide

Planning Your Network Architecture

Effective subnet planning starts with understanding your current needs and anticipating growth. Begin by inventorying all network-connected devices including workstations, servers, printers, IP phones, wireless access points, security cameras, and IoT sensors. Group devices by function, location, or security requirements. Assign each group to a separate VLAN and subnet. Use Variable Length Subnet Masking (VLSM) to allocate appropriately sized subnets rather than giving every group a /24. Document your IP addressing scheme thoroughly, including reserved ranges for network infrastructure (gateways, DNS, DHCP servers) and future expansion blocks. A well-planned addressing scheme reduces administrative overhead, improves troubleshooting speed, and prevents address conflicts as the network grows.

Subnet Design Best Practices

Follow these proven practices for robust subnet design. Use private address space (RFC 1918) for internal networks and NAT for internet access. Keep subnets small enough to limit broadcast domains but large enough to accommodate growth. A maximum of 250 to 500 devices per subnet is recommended to maintain acceptable broadcast traffic levels. Assign consistent gateway addresses (such as always using .1 or .254) across all subnets for ease of management. Reserve address ranges within each subnet for infrastructure (.1 to .10), static assignments (.11 to .49), and DHCP pools (.50 to .200). Use /30 or /31 subnets for point-to-point router links to conserve addresses. Our bandwidth calculator can help you assess throughput requirements between subnets.

Subnetting for Cloud and Virtualization

Cloud platforms like AWS, Azure, and Google Cloud require careful subnet planning within Virtual Private Clouds (VPCs). Each availability zone typically needs its own subnet, and you must account for the cloud provider's reserved addresses (AWS reserves 5 per subnet). Plan larger subnets than you think you need because expanding a cloud subnet after deployment can be disruptive. Use a /20 or /21 per availability zone for production workloads, a /24 for management networks, and /28 for load balancers or NAT gateways. In containerized environments using Kubernetes, each pod gets its own IP address, so plan for potentially thousands of addresses per cluster using /16 or larger pod network CIDRs.

Common Mistakes to Avoid

• Using the same subnet for everything: Putting all devices on a single large subnet creates excessive broadcast traffic, eliminates security boundaries, and makes troubleshooting difficult. Segment your network into functional or departmental subnets even if you have enough addresses in one block.
• Not planning for growth: A subnet that perfectly fits your current device count will run out of addresses as soon as a new device is added. Always allocate at least 30% to 50% more addresses than your current requirement. Resubnetting an established network is disruptive and error-prone.
• Confusing subnet mask and wildcard mask: The subnet mask (255.255.255.0) and wildcard mask (0.0.0.255) are inverses of each other but used in different contexts. Using a subnet mask where a wildcard mask is required (such as in a Cisco ACL) will produce unexpected filtering behavior that is difficult to debug.
• Overlapping subnet ranges: When creating multiple subnets from a larger block, ensure they do not overlap. For example, if you assign 192.168.1.0/25 and 192.168.1.0/26, the /26 is entirely contained within the /25, causing routing conflicts. Always verify that the next subnet starts after the broadcast address of the previous one.
• Forgetting reserved addresses: Every standard subnet loses at least 2 addresses (network and broadcast). Cloud providers may reserve additional addresses. A /30 has only 2 usable addresses, a /31 has 2 (point-to-point only), and a /32 is a single host address used for loopbacks. Always account for these when calculating capacity.

Frequently Asked Questions