CCNA Plan

VLSM calculator

Short answer

VLSM (variable-length subnet masking) carves one address block into subnets of different sizes so that a 100-host LAN, a 50-host LAN and a 2-host WAN link do not all waste a /24. Sort requirements largest-first, give each the smallest power-of-two block that fits hosts + 2 (network and broadcast), and start each subnet where the previous one ended.

SubnetHosts needed
NameNeededNetworkMaskUsableBroadcast
Sales100192.168.10.0/25255.255.255.128192.168.10.1–192.168.10.126 (126)192.168.10.127
Engineering50192.168.10.128/26255.255.255.192192.168.10.129–192.168.10.190 (62)192.168.10.191
Management20192.168.10.192/27255.255.255.224192.168.10.193–192.168.10.222 (30)192.168.10.223
WAN link2192.168.10.224/30255.255.255.252192.168.10.225–192.168.10.226 (2)192.168.10.227

How this works

Requirements are sorted by host count descending. Each subnet gets a block of 2^k addresses where 2^k ≥ hosts + 2, which yields prefix 32 − k. Subnets are placed back to back from the base network; if the total exceeds the base block, the tool reports that the requirements do not fit instead of silently overlapping. This is the same procedure the 200-301 IP Connectivity objectives expect you to perform by hand.

Worked examples

Base 192.168.10.0/24 with Sales 100, Engineering 50, Management 20, WAN 2 → Sales 192.168.10.0/25 (126 hosts), Engineering 192.168.10.128/26 (62), Management 192.168.10.192/27 (30), WAN 192.168.10.224/30 (2). 192.168.10.228–.255 remain free.

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Primary references

  1. RFC 4632 — Classless Inter-domain Routing (CIDR) — IETF · government · retrieved 2026-09-21, verified 2026-09-21
  2. CCNA Exam v1.1 (200-301) Exam Topics — Cisco Systems · primary · retrieved 2026-09-21, verified 2026-09-21