ToolsNetworking

Address plan checker

Paste the address ranges of your VNets, VPCs and subnets. See which ones clash, how much space is still free, and where the next block could go, before you peer, connect or merge networks.

Runs entirely in your browser: your plan isn't put in the address bar or sent anywhere

Your networks

Networks, one per line

A name and a CIDR block, in either order, separated by spaces, commas, a colon or tabs (so a paste from a spreadsheet works). A name can have several blocks, as an Azure VNet can. A start–end range also works, and IPv4 and IPv6 can be mixed. Text after # or // is ignored. Up to 2,000 networks are checked; any more are left out and the page says so.

Plan inside this range (optional)

Free space is worked out inside it. Left empty, each private block you use is analysed (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).

I need a block for (optional)

A prefix length, or a host count (rounded up using the cloud rule below).

Cloud

Nothing is uploaded or put in the address bar. Internal address plans are sensitive, so this page reads what you paste inside your browser, doesn't copy it into the URL and doesn't keep it.

How this works

Each line is split into a name and one or more networks. A block with host bits set, such as 10.1.2.3/24, is moved to its real network address (10.1.2.0/24) and flagged. Every network becomes a first and last address, in whole-number (BigInt) maths, so IPv6 is exact.
The networks are sorted by address and compared. Two overlap when they share any address: identical, one inside the other, or (for ranges that aren't blocks) partial. The shared addresses are shown as CIDR blocks.
Inside the parent range, the addresses no network uses are listed as the largest aligned blocks, which is how you would write them in a portal. "Where can I put a /N" lists the first aligned blocks of that size that are entirely free.
The findings come from the addresses themselves: IANA's special-purpose registries (RFC 6890 and updates), RFC 1918, RFC 4193, RFC 6598, and a few well-known default ranges.

What's kept: nothing. Your plan isn't stored, isn't sent to abbott.cloud or anyone else, and isn't copied into the page address.

Limitations:

It checks the addresses you paste. It can't see other networks that exist in your clouds, on premises or at partners.
Up to 2,000 networks are checked, 500 overlapping pairs and 200 free blocks are listed (the totals are always exact), and lines over 2,000 characters are skipped.
Reserved-address rules apply to IPv4 subnets. IPv6 sizes are shown in full. Some cloud services reserve more addresses or need larger subnets than the general rule: check their documentation.
The map is drawn to a linear scale, so small networks in a large range are merged into one block to stay readable. The tables have every network.

Code: addressplan.js (this page), lib/addressplan.js (parsing, overlaps, free space), lib/subnet.js and ip.js (address maths).