How to Use This Tool
Subnetting is arithmetic that looks like magic because it happens in binary while you are reading decimal. The whole subject reduces to one question: where does the mask cut the 32-bit address? Everything else — network address, broadcast, usable range, host count — falls out of that single boundary. This tool shows you the boundary as well as the answers.
Entering an address
Type anything that identifies a block and the tool will parse it: 10.0.0.0/8,
192.168.1.55/24, or an address and dotted mask like
172.16.5.1 255.255.240.0. If you enter a host address rather than the network address, that is
fine and often more useful — the tool works out which subnet that host belongs to, which is exactly
what you need when you are staring at a device's configuration and trying to work out its range.
Why two addresses are always unusable
In every IPv4 subnet, the address with all host bits set to zero is the network identifier and the
address with all host bits set to one is the broadcast address. Neither can be assigned to a device. That
is why a /24 gives 254 usable hosts rather than 256, and it is why the formula is
2^(32−prefix) − 2. The two exceptions are /31, which by RFC 3021 gives two usable addresses
for point-to-point links, and /32, a single host route.
The wildcard mask
Cisco access lists, OSPF network statements and several firewall syntaxes use a wildcard mask, which is
the bitwise inverse of the subnet mask. A /26 has mask 255.255.255.192 and wildcard
0.0.0.63. Typing the subnet mask where a wildcard is expected is one of the most common
configuration errors in networking, and it usually fails silently by matching the wrong range rather than
throwing an error.
Splitting a block
The splitter at the bottom divides your block into equal subnets of a chosen prefix and lists each one's range. This is the everyday task when you are handed a /20 for a new site and need to allocate VLANs, or carving an AWS VPC across availability zones. Plan the largest subnet first: allocating small blocks early fragments the space and leaves you unable to fit a large one later.
