Reading the output
Network is the first address in the block, with every host bit set to zero. Broadcast is the last, with every host bit set to one. Neither is assignable to a host on a normal subnet, which is why a /24 gives you 254 usable addresses rather than 256.
Wildcard is the bitwise inverse of the mask. Cisco ACLs and OSPF network statements want the wildcard, not the mask, and mixing them up is one of the most common configuration errors in the field.
The two prefixes that break the rule
Most subnet calculators get these wrong, so it is worth stating explicitly.
A /31 has two addresses and, under RFC 3021, both are usable. Point-to-point links have no need for a broadcast address, so reserving one wastes half the block. This tool reports 2 usable hosts for a /31, not 0.
A /32 is a single host route — one address, and it is usable. Loopbacks and host routes rely on this.
The four numbers worth memorising
If you know these, you can do most subnetting in your head:
/24→ 256 addresses, mask255.255.255.0/25→ 128, mask.128/26→ 64, mask.192/27→ 32, mask.224
Each additional bit halves the block. The mask octet is always 256 minus the block size, which is why the sequence runs 128, 192, 224, 240, 248, 252, 254, 255.
Working out which subnet an address falls in
The block size tells you the boundaries. For a /26, the block size is 64, so the networks are .0, .64, .128 and .192. An address ending in .100 falls in the .64 network, because 64 is the largest boundary below 100.
That single trick answers most subnetting questions faster than a calculator does.
Private ranges
The tool flags whether an address falls inside a range reserved by RFC 1918 or is otherwise non-routable:
10.0.0.0/8172.16.0.0/12192.168.0.0/16127.0.0.0/8— loopback169.254.0.0/16— link-local
If you are planning addressing for a network that may later merge with another, avoid 192.168.0.0/24 and 192.168.1.0/24. They are the default on almost every consumer router, and they collide constantly during acquisitions and VPN builds.