How CIDR notation works
A CIDR block is an address followed by a prefix length, as in 192.168.1.0/24. The prefix length counts how many leading bits identify the network; the rest identify hosts within it.
A /24 fixes the first 24 bits, leaving 8 bits for hosts, which gives 256 addresses. Each step down doubles the size: a /23 holds 512, a /22 holds 1,024. Each step up halves it. The address you enter does not need to be the network address -- 10.0.0.5/8 is understood as the /8 that contains it.
Why usable hosts is two less than the total
In an IPv4 subnet the first address identifies the network itself and the last is the broadcast address, so neither can be assigned to a host. A /24 has 256 addresses but 254 usable ones.
This is why very small subnets behave oddly. A /30 has four addresses and only two usable, which is exactly enough for a point-to-point link. A /31 is a special case defined by RFC 3021: it has no network or broadcast address, and both of its two addresses are usable. A /32 is a single host.
Private ranges and why they matter
Three IPv4 ranges are reserved for private use: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Addresses in them are not routable on the public internet, which is why they can be reused inside every organisation simultaneously.
The tool reports whether a network falls inside these ranges. Planning around them matters most when networks later need to talk to each other -- two sites that both chose 192.168.1.0/24 cannot be joined by a VPN without renumbering one of them. Picking an unusual range from 10.0.0.0/8 avoids that.
IPv6 differences
IPv6 has no broadcast address, so the network/broadcast subtraction does not apply and every address in a block is usable. Subnets are also vastly larger: the standard /64 allocation for a single network segment holds more addresses than the entire IPv4 internet.
The convention is that a site receives a /48 and each network segment within it gets a /64. Subnetting smaller than /64 breaks stateless address autoconfiguration and is generally avoided.
Frequently asked questions
What subnet size do I need for 500 hosts?
A /23, which gives 512 addresses and 510 usable. A /24 provides only 254 usable addresses and would be too small.
Can I enter an address that is not the network address?
Yes. Input is interpreted loosely, so 10.0.0.5/8 is understood as the /8 block containing that address, and the true network address is reported back.
What is the wildcard mask for?
It is the netmask inverted, and several vendors' access control lists -- Cisco's most notably -- expect it in that form rather than as a conventional netmask.
Does this send anything over the network?
No. It is pure arithmetic performed on the value you enter. Nothing is looked up or contacted.