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Next-Available Subnet

Find next available subnets from pools after subtracting existing allocations.

About This Tool

The Next Available Subnet Finder automates the process of discovering free IP address space within network pools. By analyzing existing allocations and identifying gaps, this tool suggests the next available subnets that match your requirements, eliminating the manual effort of scanning through address space. It supports intelligent allocation strategies (first-fit vs. best-fit), flexible sizing (by prefix length or host count), and handles both IPv4 and IPv6. This tool is essential for automated network provisioning, IPAM workflows, and efficient subnet management.

How to Use

  1. Enter one or more address pools in the "Pools" field, one per line (e.g., 192.168.0.0/16)
  2. Enter existing allocations in the "Allocations" field, one per line
  3. Specify desired subnet size using either "Desired Prefix" (e.g., 24 for /24) or "Host Count"
  4. For IPv4, enable "Usable hosts" to account for network and broadcast addresses
  5. Choose allocation strategy: "First fit" (fastest match) or "Best fit" (least waste)
  6. Set "Max" to limit the number of candidate subnets returned
  7. Click "Find" to discover available subnets
  8. Review candidate subnets with their parent pools, total size, and usable host counts

Features

  • Automatically finds available subnets in address pools
  • Supports both IPv4 and IPv6 address families
  • Flexible sizing: specify by prefix length or host count
  • Two allocation strategies: first-fit and best-fit
  • Accounts for IPv4 network and broadcast addresses when needed
  • Handles multiple pools and existing allocations
  • Returns multiple candidate subnets (configurable limit)
  • Shows parent pool, size, and usable hosts for each candidate
  • Real-time calculation as parameters change

Common Use Cases

  • Automating subnet allocation in IPAM systems
  • Finding space for new VLANs or network segments
  • Provisioning subnets for new cloud environments (AWS VPCs, Azure VNets)
  • Allocating address space for new branch offices
  • Discovering capacity for customer allocations in service provider networks
  • Planning data center rack or row subnet assignments
  • Finding space for Kubernetes pod networks
  • Allocating subnets for new data centers or regions
  • Identifying gaps for subnet expansion
  • Automated network provisioning workflows

Technical Details

The Next Available Subnet finder calculates free address space by subtracting existing allocations from available pools, then identifying appropriately-sized gaps that can accommodate the requested subnet size.

Allocation Strategies:

  • First-fit: Returns the first available subnet that meets the size requirement. Fastest, good for sequential allocation patterns.
  • Best-fit: Returns subnets from the smallest available gap that can fit the request. Minimizes fragmentation and waste.

Example - First Fit:

  • Pool: 192.168.1.0/24
  • Allocated: 192.168.1.0/26, 192.168.1.64/26
  • Request: /26 (64 addresses)
  • Result: 192.168.1.128/26 (first available /26 in gaps)

Example - Best Fit:

  • Pool: 10.0.0.0/22 with various allocations creating gaps of sizes /26, /25, /24
  • Request: /26
  • Result: Subnet from the /26 gap (perfect fit, no waste)

Prefix vs. Host Count:

You can specify subnet size in two ways:

  • Desired Prefix: Directly specify /24, /26, /64, etc.
  • Host Count: Specify number of hosts needed (e.g., 50). Tool calculates required prefix automatically.

IPv4 Usable Hosts:

When enabled for IPv4, the tool accounts for network and broadcast addresses:

  • Request 50 usable hosts → Requires /26 (64 total, 62 usable)
  • Request 62 usable hosts → Requires /25 (128 total, 126 usable)

Algorithm Steps:

  1. Parse and validate all pools and allocations
  2. Calculate gaps within each pool by subtracting allocations
  3. Determine required prefix length based on host count or specified prefix
  4. Search gaps using selected strategy (first-fit or best-fit)
  5. Return up to Max candidates with metadata (parent pool, size, usable hosts)

Performance and Scalability:

The tool efficiently handles large pools and many allocations by using optimized data structures. For extremely large IPv6 pools, best-fit may take longer than first-fit but provides better utilization.