Subnet & VLAN Calculator
Get the network, broadcast, netmask, and host range from CIDR notation, plus a subnet and VLAN planner.
CIDR math comes up constantly in triage: scoping an alert to a subnet, checking whether a beaconing host shares a /24 with a domain controller, working out how many addresses a firewall rule really covers. It is also easy to be off by one doing it by hand.
This runs in your browser. Nothing you type or open is uploaded.
How to read it
Calculate gives the network, broadcast, masks, and host range for an IPv4 or IPv6 block. Add a second address to check whether it is in the same subnet. Special-use ranges, such as RFC 1918 or documentation space, are flagged.
A /31 has no network or broadcast split; RFC 3021 makes it a two-address point-to-point link. A /32 is one host.
Equal split carves a network into same-size subnets. Custom sizes plans subnets largest first for the host counts you give. Supernet finds the smallest block that covers a list.
Sources
- RFC 4632: Classless Inter-domain Routing (CIDR). The authoritative IETF specification for CIDR notation and address aggregation that this page's own arithmetic implements. Worth consulting directly for the full addressing/aggregation rules beyond what a calculator surfaces.
- RFC 1878: Variable Length Subnet Table For IPv4. The original VLSM (Variable Length Subnet Masking) reference this page's own "Custom sizes (VLSM)" planner mode implements: includes the full subnet-size/host-count table for every prefix length.
Worth using too
- Visual Subnet Calculator. A free interactive tool that visually splits and joins subnets within a base network as you click, useful for cross-checking this page's own VLSM planner output, or for exploring "what if" splits before committing to a segmentation plan.