VLSM Calculator & Config Generator — address plan to Cisco / YAMAHA RTX config
List segments with host counts and VLANs; get a largest-first VLSM plan with gateways, usable ranges, spare capacity and free blocks, plus Cisco router-on-a-stick, L3 switch SVI, DHCP and YAMAHA RTX config and a CSV.
🔒 Runs entirely in your browser. Your input and output are never sent to a server or stored — fully private.
Address plan
Configuration
VLSM gives each segment the smallest block that fits, allocating the largest first so every block stays aligned. Leave growth room: a LAN at 90% of its block today will need renumbering soon, so round host counts up before planning.
About this tool
This VLSM calculator turns a list of segments into a working address plan and the configuration to build it. Enter a base network and one line per segment — name, number of hosts and optionally a VLAN ID — and it gives each segment the smallest block that fits, allocating the largest first so every block stays aligned. You see the network, mask, gateway, usable range, broadcast and spare capacity of every segment, the share of the base you have used and the free blocks left over. It then writes configuration skeletons for a Cisco router with dot1Q sub-interfaces, a Cisco layer-3 switch with SVIs, DHCP pools and YAMAHA RTX, plus a CSV for your documentation. Everything runs in your browser.
Frequently asked questions
What is VLSM and why not just use equal-sized subnets?
Variable-length subnet masking lets each segment have its own prefix length instead of one size for all. Fixed-size subnetting must size every subnet for the largest one: a /24 split for a 100-host LAN gives only two /25s, so the 50-, 25- and 10-host LANs and a router link do not fit. With VLSM the same /24 holds a /25, /26, /27, /28 and /30 — 244 of 256 addresses used — which is the default example in this tool.
Why allocate the largest subnet first?
Every block must start on a multiple of its own size: a /26 can only begin at .0, .64, .128 or .192. If you place the largest blocks first, each next block automatically starts on a boundary it can use, so nothing is wasted between them. Placing a /30 first at .0 would force the following /25 to jump to .128, leaving .4 to .127 as a fragmented hole that only smaller blocks can fill. The tool always sorts by block size, then keeps your input order for equal sizes.
Should the host count include the router's gateway address?
The gateway uses an address like any host, and forgetting it is the classic off-by-one in subnetting exams and real plans. A /26 has 64 − 2 = 62 usable addresses: 62 PCs plus a gateway is 63, which needs a /25. By default the tool adds one address for the gateway to every LAN; untick that if your counts already include it. Router links of one or two hosts without a VLAN are treated as point-to-point and get no extra address.
Should router-to-router links use /30 or /31?
A /30 has four addresses, two of them network and broadcast, so half is wasted. RFC 3021 allows a /31 on point-to-point links, where both addresses are usable because there is no broadcast to reserve, halving the space consumed by links. Most current routers and Linux support it, but some older platforms, firewalls and monitoring tools still expect /30, so both ends must agree. The /31 option in this tool applies only to two-host links without a VLAN.
Router-on-a-stick or layer-3 switch — which config should I use?
Router-on-a-stick trunks every VLAN to one router port and routes between them on dot1Q sub-interfaces; it is simple and fine for small sites, but all inter-VLAN traffic shares that one link. A layer-3 switch routes between VLANs in hardware on SVIs (interface VlanN), which scales far better and leaves the router for WAN and policy. On YAMAHA RTX, tagged VLANs are created as lan1/1, lan1/2 and so on with vlan … 802.1q vid=, and the generated DHCP scopes include the gateway. All output is a skeleton: add trunk, security and routing settings for your design.