VLSM allocation from a 10.10.0.0/16 block · Non-overlapping subnets · Usable hosts & address waste
This simulator performs Variable Length Subnet Masking (VLSM) allocation across a 10.10.0.0/16 supernet, assigning non-overlapping subnets to each VLAN using a largest-first packing algorithm. Network engineers use it to visualize address space consumption, confirm zero subnet overlap, and document IP address plans before configuring switches and routers.
VLSM (Variable Length Subnet Masking) allows different subnets within a network to have different prefix lengths, matching each subnet exactly to its host count requirement rather than wasting addresses on equal-size blocks. For each VLAN, the smallest prefix whose usable host count (2^(32−prefix) − 2) meets or exceeds the required hosts is selected. The −2 accounts for the network address (all host bits zero) and broadcast address (all host bits one), which cannot be assigned to hosts.
The largest-first allocation strategy packs large subnets first to minimize fragmentation within the address pool. Subnets are aligned to their natural block boundaries (a /24 must start on a multiple of 256, a /22 on a multiple of 1024) to ensure valid routing. The total allocated addresses and waste are tracked to help optimize prefix selection.
RFC 1918 defines the private IPv4 address ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) used for internal networks. RFC 4632 defines CIDR (Classless Inter-Domain Routing) notation and VLSM. IEEE 802.1Q governs VLAN tagging on trunk links, supporting VLAN IDs 1–4094. IEEE 802.1p extends 802.1Q with CoS priority bits. Inter-VLAN routing is performed by a Layer 3 switch (SVI — Switched Virtual Interface) or router-on-a-stick configuration per RFC 1812. DHCPv4 scope assignments per RFC 2131 must align with the subnet boundaries calculated by this tool.
Assign VLAN IDs that reflect your network hierarchy: 10 for users, 20 for servers, 30 for voice, 40 for IoT, 99 for management is a widely used convention that aids troubleshooting. Reserve 20–30% of each subnet for growth before the next refresh cycle — size to the next prefix up if utilization would exceed 70%. Keep voice (VoIP) traffic in a dedicated VLAN to enable QoS marking at the access switch without needing to inspect traffic. Management VLAN (for switch, AP, and UPS access) should be a small, tightly controlled subnet separate from user VLANs, with ACLs restricting access to management interfaces.
Add VLANs for each logical segment in your network, entering the VLAN ID and the number of hosts required. The visualizer allocates CIDR subnets from the 10.10.0.0/16 block using largest-first VLSM and displays the network address, prefix, and usable host range for each VLAN. Check the address waste column — if waste exceeds 50% of the subnet, consider using a smaller prefix. The status banner confirms zero overlap and tracks overall allocation efficiency. Use the allocation table to populate your IP address management (IPAM) system.
Every IPv4 subnet reserves the first address as the network address (identifies the subnet itself) and the last address as the broadcast address (a packet sent here reaches all hosts in the subnet). A /24 subnet (256 total addresses) therefore has 254 usable host addresses, not 256.
An access port carries traffic for a single VLAN and is connected to end devices (PCs, phones, cameras). A trunk port carries tagged traffic for multiple VLANs using 802.1Q headers and is used for switch-to-switch and switch-to-router uplinks. End devices never see VLAN tags on access ports.
A /24 subnet has 256 addresses (2^8), with 254 usable host addresses after reserving the network and broadcast addresses. A /23 supports 510 hosts, /22 supports 1,022 hosts, /25 supports 126 hosts, and /26 supports 62 hosts.
No. Each VLAN must have a unique non-overlapping subnet. Overlapping subnets cause routing ambiguity and are a common misconfiguration that results in intermittent connectivity between VLANs. This visualizer enforces non-overlapping allocation automatically.
Use Layer 3 switch SVIs for inter-VLAN routing between trusted segments (user to server VLANs) where speed and latency matter. Use a firewall for inter-VLAN routing between untrusted segments (guest, IoT, internet) where stateful inspection, IPS, and ACL enforcement are required.
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