Subnet Calculator Guide
Identify one network, or split a parent block into several prefix lengths, without guessing the next boundary.
rookvpn.com/subnet turns an address and a prefix length into a network range. It does not program a router and it does not know which other site already uses an address.
IPv4
Enter the address and the prefix you were given, not the prefix you wish you had. For 192.168.10.40/24 the network is 192.168.10.0, the broadcast is 192.168.10.255, and the usable hosts on an ordinary LAN are .1 through .254. A /32 is one host. A /31 is a point-to-point link that does not spend an address on broadcast the way a /24 does. Do not subtract a broadcast the link does not use.
The next subnet starts on a boundary. A /28 that begins at .0 ends at .15. The following block begins at .16. If a second range also contains .10, you assigned the same host twice. The calculator will not shout about a VLAN you typed on a different day.
IPv6
IPv6 subnets do not have a broadcast address. A /64 is the normal LAN. A /128 is one host. :: is one compressed run of zeros. Do not carve a LAN into /120s unless you can name the constraint that forced it.
If a leak test showed an ISP IPv6 prefix while your VPN only carries IPv4, that is a tunnel problem on rookvpn.com/check/vpn-leak-test/ipv6. The calculator explains the prefix. It does not disable IPv6 for you.
VLSM
Split a block you actually own. List the sizes largest first so each child lands on a legal boundary and the remainder stays contiguous. A /24 that must yield a /26, a /27, and a /28 gets the /26 at the start of the parent. Placing the /28 at .0 first leaves the /26 with nowhere legal to sit.
Copy each resulting prefix into the IPv4 calculator and read it back before you type it into a device.