Inputs
Example
192.168.10.42/24 with a /26 split shows exact network math and a capped preview; /31 follows RFC 3021, while IPv6 has no broadcast address.
CIDR/Subnet Calculator
Calculate exact IPv4 and IPv6 networks and BigInt address counts locally, including RFC 3021 /31, host /32, embedded IPv4, and capped split previews.
Use it for quick network math without sending internal ranges or address plans to a server.
Address math that has to be exactly right
Subnet arithmetic is unforgiving in a way little else in networking is: an off-by-one in a boundary or a mask misread by one bit ends up in a firewall rule, a routing table, or a DHCP scope, and produces the sort of outage that gets diagnosed at midnight. The math itself is mechanical — which is precisely the argument for not doing it in your head during a change window.
This calculator takes an IPv4 or IPv6 CIDR block and returns the facts a plan is built from: exact network boundaries and address counts, IPv4 broadcast or RFC 3021 point-to-point roles, first and last addresses, and IPv4 netmask and wildcard mask. Given a target prefix, it also previews a bounded sample of smaller subnets — the everyday operation of carving a /24 into /26s or a /48 into /64s, done by rules instead of by habit.
From one block to a subnet plan
Enter the address with its prefix and read the boundaries first — network plus first and last address. For IPv4, the broadcast or /31-/32 role and wildcard mask are explicit; IPv6 correctly has no broadcast field. IPv4 and IPv6 are detected automatically, including embedded dotted IPv4 in IPv6; forcing the family is only needed for ambiguous shorthand.
Add a target prefix when you need the split. The preview is intentionally capped: a /16 into /29s is over eight thousand subnets, and IPv6 splits reach numbers with no operational meaning — the first entries plus the per-subnet size are what a plan actually requires. For allocation work, note the counts more than the list: how many subnets the split yields and how many addresses each carries is the decision-relevant pair.
Local arithmetic, deliberately unbounded families
Every calculation runs locally in the browser — no address, subnet plan, or split request is sent anywhere by this tool, which makes it safe for private and internal addressing plans. IPv6 math uses integer arithmetic throughout, so large prefixes stay exact rather than floating-point-approximate; the subnet list display is capped to keep the browser responsive. The calculator computes address structure — it does not know your routing policy, VLAN layout, or reservation conventions, and it cannot tell you whether a technically valid split is operationally wise.
Frequently asked questions
Why are /31 and /32 IPv4 networks treated specially?
Because the traditional network-plus-broadcast model does not apply: a /31 is a two-address point-to-point link per RFC 3021, and a /32 is a single host. The calculator reports them that way instead of pretending they contain unusable addresses.
Why is the subnet list capped?
Because splits scale exponentially — thousands of rows for modest IPv4 cases, astronomically more for IPv6. The first subnets establish the pattern; rendering the rest would freeze the page without adding information.
Does IPv6 have a broadcast address?
No. IPv6 replaced broadcast with multicast, so the result shows first and last address of the block instead. The last address is a perfectly ordinary assignable address in IPv6.
What is the wildcard mask for?
It is the bit-inverted netmask, used by ACL syntax on many routers and firewalls. Having it computed next to the netmask removes the manual inversion step where ACL errors classically originate.
Why does a /64 seem excessive for one LAN?
Because IPv6 was designed around subnet size uniformity rather than address conservation: SLAAC requires a /64, and features across the ecosystem assume it. Address scarcity thinking from IPv4 mostly does not transfer.
Is it safe to enter private network plans?
Yes, for this calculator: the math runs entirely locally and nothing is transmitted. The general caution stands — keep internal plans out of URLs, analytics, and third-party tools — and this page is built to respect exactly that.