IP Subnet Calculator — IPv4 Network Mask & Range Splitter

How to Use the IP Subnet Planner:

1. Enter the Parent Network CIDR: Type or paste an IPv4 CIDR into the Network CIDR field. The format is an IP address followed by a slash and prefix length, such as 192.168.1.0/24 or 10.0.0.0/16. The tool auto-calculates subnet details as you type — no button press needed.

2. Review the Parent Subnet Details: Once a valid CIDR is entered, the Parent Subnet section shows network address, broadcast address, first and last usable host, subnet mask, wildcard mask, total IPs, and usable host count. Tags show the IP class and whether the network is private (RFC 1918) or public.

3. Enter a Split Count (Optional): To divide the parent network into equal subnets, type a number between 2 and 512 in the "Split into N subnets" field. The tool automatically selects the smallest prefix that can accommodate N subnets — for example, splitting a /24 into 4 subnets produces 4 × /26.

4. Read the Subnet Split Table: The split table lists each subnet with its CIDR, network address, first host, last host, broadcast address, and usable host count. The table header shows how many subnets are displayed versus how many are possible (since the tool rounds up to the next power of 2 and shows only the N you requested).

5. Copy Any Value: Hover over any address cell in the details grid and click the copy icon that appears. In the split table, click any cell directly to copy its value to the clipboard.

6. Use Quick Examples: Click any example button to load a preset CIDR and split count. Examples cover a home LAN ÷ 4, an office /16 ÷ 8, data centre VLANs, a Class A divided into 256 subnets, and a multi-site /20 split into sixteen /24s.

7. Understand the Prefix Math: When splitting into N subnets, the tool calculates bits_added = ceil(log2(N)) and adds that to the parent prefix. Three subnets requires 2 extra bits (2² = 4 possible, 3 shown). The table header always tells you the actual possible count so you know how many subnets are left unallocated.

8. Clear and Start Over: Click Clear to reset both inputs and start fresh with a different network.

Common Use Cases:

• VLAN planning: Divide a corporate /16 into department-sized /24 or /22 subnets.
• Cloud VPC design: Split an AWS or Azure VPC CIDR into availability zone subnets.
• Home lab setup: Divide a 192.168.0.0/24 into separate segments for devices, IoT, and servers.
• Data centre rack allocation: Allocate /27 or /28 blocks from a larger pool to server racks.
• ISP address management: Split provider blocks into customer allocations.
• Kubernetes networking: Plan pod and service CIDRs from a parent range.
• Firewall rule writing: Look up network and broadcast addresses to write accurate ACLs.
• Study and certification prep: Visualise subnet boundaries for CCNA, CCNP, or Network+ exams.

Tips and Best Practices:

• The tool rounds N up to the next power of 2 — splitting into 5 subnets uses a /3 block (8 possible), showing only 5.
• A /24 (254 hosts) is the most common LAN block; split it into 2 = two /25s (126 hosts each).
• Use the wildcard mask directly in Cisco ACLs and OSPF network statements.
• For /31 links (RFC 3021), both addresses are usable and there is no broadcast — ideal for router-to-router links.
• To create subnets of different sizes (true VLSM), run multiple calculations: first allocate the largest subnets, then use the remaining space for smaller ones.
• Private ranges (RFC 1918): 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 — these are not routed on the public internet.
• When designing cloud subnets, reserve at least one subnet per availability zone plus a spare for future growth.
• The maximum displayable split is 512 subnets; for larger allocations run multiple smaller splits.