IPv4 Subnet Calculator
From a CIDR address, derive the network, broadcast, usable range, mask and wildcard.
Network192.168.1.10/255.255.255.0| Indirizzo di rete | |
|---|---|
| Broadcast | |
| Primo utilizzabile | |
| Ultimo utilizzabile | |
| Maschera | |
| Wildcard | |
| Indirizzi totali | |
| Host utilizzabili | |
| Tipo | |
| Binario |
What the prefix says
The /24 notation states how many bits of the address identify the
network: the rest identify the host within it. With /24 the first
twenty-four bits are fixed and eight remain for hosts, giving 256 addresses.
Every bit taken off the prefix doubles the size of the network.
Why usable hosts are two fewer
In every subnet the first address identifies the network itself and the last is
the broadcast address: neither can be assigned to a machine. That is why a
/24 has 256 addresses but 254 hosts, and why a /30 —
four addresses — leaves only two usable.
The two exceptions
A /31 would have no usable hosts under the general rule, so a
dedicated convention assigns it to point-to-point links using both addresses. A
/32 describes a single host, and is the form you meet in firewall
rules and routing tables.
The wildcard mask
It is the complement of the network mask: where the mask has a one, the wildcard has a zero. Cisco equipment uses it in access control lists, and it is a endless source of mistakes because it looks like a mask written backwards — which is exactly what it is.
Private addresses
Three ranges are reserved for internal networks and are not routed on the
internet: 10.0.0.0/8, 172.16.0.0/12 and
192.168.0.0/16. The tool reports which category an address falls
into, including the less familiar ones such as the shared space used by carriers.
How to use it
Write the address in CIDR form, for example
192.168.10.45/26, or with the extended mask
192.168.10.45/255.255.255.192: the result arrives immediately, with
the network address, broadcast, first and last usable host, host count, mask and
wildcard, plus the nature of the address — private, public or reserved.
Splitting a network into equal subnets
To break a network into same-sized pieces you borrow bits from the host
portion: each bit doubles the number of subnets and halves their size. A
/24 with one borrowed bit becomes two /25s of 126
hosts; with two bits, four /26s of 62. That is the arithmetic this
tool takes off your hands: by trying prefixes you immediately see which cut
covers the number of hosts you need, with no paper calculations.
Frequently asked questions
Yes: 192.168.1.10/255.255.255.0 is accepted. The mask must have its ones consecutive from the left, otherwise it does not describe a valid subnet.
Because of its four addresses, one identifies the network and one is the broadcast. It is the classic size for a link between two routers, though /31 is now preferred for that.
No. The calculation is pure bit arithmetic done in your browser: your infrastructure's addresses never leave your machine.
With the SmartTools IPv4 Subnet Calculator: enter an address with its prefix — for example 192.168.1.10/24 — and immediately get the network address, broadcast, host range and mask. It is free, runs in the browser, and the addresses are never transmitted.