Subnet Calculator — IPv4, IPv6 & VLSM

Subnetting by hand means binary, and one slipped bit puts a server on the wrong network. Type an address with its prefix, and this gives the network, broadcast, usable range and both masks, then shows the bits so you can see why.

It opens on 192.168.10.57/26, a host in the network 192.168.10.0/26, with 62 usable hosts from 192.168.10.1 to 192.168.10.62 and a broadcast of 192.168.10.63.

How to Use the Calculator and VLSM Planner

1. Type 192.168.1.10/24, 192.168.1.10 255.255.255.0 or an IPv6 address such as 2001:db8::1/48, or drag the prefix slider.
2. Read the results, each with its own copy button. Network bits are orange in the binary view.
3. Use Split to cut an IPv4 block into equal subnets.
4. In the planner, enter the block and one group per line, such as Sales 60, and copy the plan as CSV.

The planner is the part worth knowing about. Equal subnets waste space when one team has 60 devices and a router link has 2, so VLSM gives each group the smallest block that fits, placing the largest first so every block starts on its own boundary. The sample needs 60, 28, 12, 6 and 2 hosts from a /24, and gets a /26, /27, /28, /29 and /31, using 122 of the 256 addresses.

The Method: Masks and Host Counts

A prefix says how many leading bits name the network. The network address is the IP with the host bits cleared, the broadcast is the same with them all set, and a /n holds 2³²⁻ⁿaddresses. Two of those are normally reserved, so a /26 has 64 addresses and 62 usable hosts. The wildcard mask is the mask inverted, as used in Cisco access lists. These results were compared with Python’s ipaddress library on 30,000 random IPv4 and 30,000 IPv6 networks, plus 3,000 splits and 3,000 VLSM plans.

Edge Cases: /31, /32 and IPv6

A /31 has no broadcast: both addresses are hosts on a point-to-point link (RFC 3021). A /32 is a single address. A mask must have contiguous ones, so 255.255.0.255 is refused. Leading zeros such as 010.0.0.1 are rejected too, because some systems read them as octal. The class shown is historical, since routing has ignored classes since CIDR. In the planner, a list that does not fit names the group that ran out of room, for example “doesn’t fit… Use a larger block.” Splitting lists the first 256 subnets and counts the rest. Splitting and planning work on IPv4 only.