| # | Network | Host range | Broadcast | Hosts |
|---|
The split above cuts a block into equal pieces. Real networks are not equal: 40 desks, 12 phones, a pair of router interfaces. Type the host counts and this allocates largest first, on the boundary each subnet has to start on.
One prefix per line. This finds the smallest single prefix covering all of them — and, more importantly, tells you whether that prefix also covers addresses you did not list.
An IPv4 address is 32 bits, usually written as four octets (0–255 each). CIDR notation like
/24 says how many of those leading bits are the network — fixed for everyone on the
subnet — leaving the rest for individual hosts. A /24 fixes 24 bits and leaves 8, giving
256 addresses (254 usable). Move the slider and watch the block grow or shrink by powers of two.
Take 192.168.1.100/24. The /24 mask
(255.255.255.0) keeps the first three octets, so the network is
192.168.1.0 and the broadcast is 192.168.1.255. Usable hosts run
.1 to .254 — 254 of them.
Split that /24 into /26s and you get four subnets of 62 hosts each:
.0, .64, .128 and .192. That's how a flat network gets divided between, say, four VLANs.
Every block reserves its first address as the network identifier and its last as the broadcast address, so a /24's 256 addresses give 254 usable hosts. The exceptions are /31 (a two-address point-to-point link) and /32 (a single host).
It's the inverse of the subnet mask — 1s where the mask has 0s. Access-control lists and some routing tools (notably Cisco) use it instead of the subnet mask to describe the same range.
Three blocks are reserved for private use: 10.0.0.0/8, 172.16.0.0/12, and
192.168.0.0/16. Addresses inside them aren't routable on the public internet — they're for
your own networks.
Not yet — this is IPv4 only. IPv6 uses the same CIDR idea but with 128-bit addresses, so host counts become astronomically large and the "usable = total − 2" rule no longer applies.
IPv4 only, and strictly classless — the historical class A/B/C rules are ignored, because CIDR replaced them. Usable hosts are total minus two for the network and broadcast addresses, except at /31 (a two-address point-to-point link) and /32 (a single host). The VLSM panel deliberately uses a different mask per subnet, and allocates largest first because each subnet must begin on a multiple of its own size. Two-host subnets get a /30, not a /31: a /31 is a point-to-point link with no broadcast address. Summarising reports whether the covering prefix takes in addresses you did not list. It has no knowledge of your actual network: which addresses are already assigned, reserved for DHCP, or taken by a gateway is something only your own records can tell you.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.