IP Calc is a subnet calculator for people who do this for a living.
Type an address and the whole block appears as one figure you can read at a glance — network, usable range, broadcast, and how much space is actually in there. Drag the block to resize it and the numbers follow.
THE COUNT YOUR CLOUD ACTUALLY GIVES YOU
Textbook subnet math says a /20 has 4,094 usable addresses. Your VPC disagrees.
Switch the reserve profile to AWS, Azure or GCP and the calculator, VLSM and Split all re-read under that provider's rules: five reserved addresses per subnet on AWS and Azure, four on GCP, and the smallest subnet each will accept — /28 on AWS, /29 on Azure and GCP.
The app names the addresses it took instead of just subtracting them. Under AWS that reads network, VPC router, DNS, future use, broadcast — so you can see where the count went.
START FROM WHAT YOU NEED
You know you need room for 400 phones. Type the host count and IP Calc sizes the subnet for you: 400 hosts, /23, 510 usable.
NINE TOOLS
• VLSM Calculator — subnet a block by host count, with named allocations and the free space shown
• Subnet Split — divide a block into equal subnets
• Compare Subnets — overlap and containment, with the shared address count spelled out
• Supernet Calculator — smallest covering block, and exactly how many addresses that wastes
• IPv6 Subnet Calculator — prefix size and subnet count, /0 to /128
• Bandwidth Calculator — transfer time, rate and units
• IP Converter — decimal, hex and binary
• Wildcard / ACL Calculator — wildcard mask plus copy-ready Cisco ACL lines
• IP Validator — check one address and name its type
KEEP YOUR NETWORKS
Bookmark the blocks you care about and give them names. Every pair of bookmarks is checked for collisions, and when two of them share addresses the app says so at the top of the screen and draws exactly where the overlap falls. Recents are kept separately and never trigger the check — a scroll trail is not a plan.
HOW IT WORKS
• Everything is calculated on your device. Nothing you type leaves it.
• No account, no tracking, no network access at all.
• No ads, no subscription.
• Full VoiceOver support and Dynamic Type throughout.
• A custom keypad on every field, matched to what that field accepts, so you are never hunting for a slash or a dot.
