2001:db8::/56
every number, and where it comes from.
Network and broadcast, first and last usable host, the mask in every notation, and the seam drawn in binary. Change the address below to calculate anything else — it recomputes in your browser, without a request.
Stop counting addresses. Count the /64s.
2001:db8::/56 holds 272 addresses — ≈ 4.7 sextillion. That number means nothing. This prefix cuts into 256 /64s, and a /64 is the standard LAN: SLAAC autoconfiguration works at exactly that size, so the count of /64s is the number that actually plans a network.
One address, three jobs
128 bits, bandedEvery form of this prefix
RFC 5952 canonicalCutting it into LANs
subnet rangesThings IPv6 does not have
unlearn theseIt travels only if you copy it and send it — opening it is the one thing that does reach our access log. What you type in the box never enters the address bar.
What we computed
arithmetic, not discoveryCopy as text
Prefix: 2001:db8::/56 Expanded: 2001:0db8:0000:0000:0000:0000:0000:0000 First: 2001:db8:: Last: 2001:db8:0:ff:ffff:ffff:ffff:ffff Addresses: 2^72 (about 4.7 sextillion) /64 LANs: 256 Scope: Documentation -- calculated at status.uptimia-dev.ovh/subnet-calculator
One number, four consequences.
Most calculators make you pick a page — IPv4 here, IPv6 elsewhere. Addresses stopped working like that years ago, so this one takes either.
Where the network ends
The network address and the broadcast are not configured anywhere — they fall out of the seam. All host bits zero names the block; all host bits one calls everyone in it.
network · broadcastHow many devices fit
Host bits set the size: 232−n addresses, minus the two the block spends on itself. The famous −2 — and the /31 and /32 exceptions where it does not apply.
2^(32−n) − 2Who needs a router
Two hosts compare networks with one AND. Same result and they talk directly; different and everything goes via the gateway. A wrong mask here is why two machines on the same wire cannot see each other.
the AND testHow to grow
Every block splits into two halves, four quarters — or unequal pieces sized to real needs. That is VLSM, and it is the difference between fitting and buying more space.
split · right-sizeSix subnet mistakes people keep making
Much of subnetting folklore is a habit from 1993. What the numbers actually mean.
"No usable hosts"? Not since RFC 3021: on a link with exactly two ends, both addresses are usable. No network address, no broadcast, nothing wasted. The old habit says /30 and burns half of it.
Inside a /20, yes, and a perfectly normal one. Broadcast means all host bits set, which only the top address of the whole block has. Ending in .255 means nothing without the mask.
Not a mask. Mask bits have to be contiguous — ones, then zeros, no gaps. This tool rejects it, and so does essentially all real equipment. If a config accepts it, worry.
It is not. RFC 1918's middle range is 172.16.0.0/12, which ends at 172.31.255.255. 172.32.x.x belongs to someone on the public internet — and one day your traffic will find them.
The mask, inverted — Cisco ACL notation, where 0 means "must match" and 1 means "don't care". Same /26 boundary, opposite spelling. We print both, and accept either as input.
Classes stopped mattering in 1993 when CIDR shipped. 10.0.0.0/8 is "class A" only in exam questions — what routes is the prefix length, nothing else. We print the class because interviews still ask.
The subnets people actually look up
Worked answers for the blocks that turn up in real networks, each with a note on why it matters.
Free tools are just the start.
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