IP Subnet Calculator
IPv4 and IPv6, Worked Out For You
This IP subnet calculator takes an address and its prefix and works out the whole range at once: network, broadcast, the usable hosts, both masks and the reverse DNS zone. It handles IPv6 too, and it splits a range into smaller subnets.
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IP Subnet Calculator
IPv4 and IPv6 ranges, masks and splits, worked out in your browser
Type 10.0.0.0/8, 192.168.1.10 255.255.255.0 or an IPv6 range such as 2001:db8::/48. It recalculates as you type.
- Network
- –
- Usable hosts
- –
- Prefix
- –
- Scope
- –
| Property | Value |
|---|
Pick a longer prefix than the range above, for example /26 inside a /24, and the subnets are listed here.
| # | Subnet | First address | Last address |
|---|
Ready. Type an address with its prefix.
What An IP Subnet Calculator Tells You
A subnet is a block of addresses that share the same first bits. In /24, the 24 counts the fixed bits out of 32. So 192.168.1.0/24 fixes the first 24 bits and leaves 8 free, which gives 256 addresses.
Working that out by hand means binary arithmetic, and a single slip puts two offices on the same range. This calculator does the arithmetic for you and shows every number an engineer usually needs, in one table.
The Network Address, the Broadcast Address and the Hosts Between
The first address of an IPv4 range names the network itself, and the last is the broadcast address. No device can take either one, which leaves a /24 with 256 addresses yet just 254 hosts.
There are two exceptions. Point-to-point links often use a /31, and both of its addresses can carry traffic, as RFC 3021 allows. In the same way, a /32 covers exactly one host. The Denwix calculator handles both rules for you.
The Subnet Mask Beside Its Wildcard Mask
The subnet mask is the prefix written as four numbers, such as 255.255.255.0 for a /24. Routers and older systems still ask for it in that form.
The wildcard mask is its mirror image, 0.0.0.255 for the same range. Access lists on many firewalls and routers expect the wildcard rather than the mask, which is why both appear in the table.
Which Ranges Are Private, Public or Reserved
Some ranges never travel across the public internet. Your router hands out 10.0.0.0/8, 172.16.0.0/12 or 192.168.0.0/16 addresses, and mobile carriers sit behind 100.64.0.0/10. The calculator names the scope of every address, so a mistyped public range stands out at once.
IPv6 Without The Headache
IPv6 addresses are 128 bits long, so their ranges are enormous. A single /64, the usual size for one network, holds more addresses than the whole IPv4 internet. Therefore the calculator shows IPv6 totals as powers of two and counts how many /64 networks fit in a larger allocation.
It also writes every address in its short, standard form and in full, since logs and firewall rules often use different ones.
Splitting A Range Into Subnets
Type a longer prefix in the split box and the range is divided for you. A /24 split into /26 blocks, for instance, gives four subnets of 64 addresses, each with its first and last address listed.
On this page
- The tool
- What An IP Subnet Calculator Tells You
- Working With the IP Subnet Calculator, Start to Finish
- What Sets This IP Subnet Calculator Apart
- Rules For Planning Subnets
- Six Subnetting Mistakes That Break Networks
- Subnet Calculator Questions People Ask
- Tools For Networks And Domains
Keep going
Working With the IP Subnet Calculator, Start to Finish
One field in, the whole range out, as fast as you can type.
- 1
Type the address
Use CIDR form such as 10.0.0.0/8, or an address and a mask such as 192.168.1.10 255.255.255.0.
- 2
Read the four numbers
The network, usable hosts, prefix and scope sit at the top, where most questions end.
- 3
Check the details
The table adds broadcast, the usable span, both masks, the class and the reverse DNS zone.
- 4
Split if you need to
Enter a longer prefix to list the smaller subnets, then copy or download the lot.
What Sets This IP Subnet Calculator Apart
Most calculators stop at the mask. This one answers the next question as well.
One input box for both IPv4 and IPv6
Type either kind; the calculator recognises which it is, so there is no second tool to find.
Masks written both ways
Prefix, dotted mask, wildcard and binary all appear together, since different devices ask for different forms.
Scope named for you
Private, public, loopback, CGNAT or documentation, so a wrong range is obvious before it reaches a config file.
Reverse DNS zone included
The in-addr.arpa or ip6.arpa zone for the range, ready for whoever runs your DNS.
Subnet split built in
Divide a range into smaller blocks and get every first and last address in a table.
Nothing leaves your tab
The arithmetic runs in your browser, so internal addressing plans stay on your machine.
Rules For Planning Subnets
Eight habits that keep an addressing plan tidy as a network grows.
- Leave room to grow. Size each subnet for twice the devices you have today, since renumbering later costs far more than spare addresses.
- Keep blocks on their boundaries. A /26 should start at .0, .64, .128 or .192. Otherwise routers reject it or summarise it wrongly.
- Group by purpose. Put servers, staff, guests and devices in separate ranges, so firewall rules can follow the plan.
- Steer clear of 192.168.0.0/24 and 192.168.1.0/24. Almost every home router uses them, so a VPN from home collides with an office on the same range.
- Give router-to-router links a /31. A point-to-point link needs two addresses, and a /31 gives exactly that without wasting two more.
- Give IPv6 networks a /64. Automatic addressing expects it, and splitting a /64 further breaks features devices rely on.
- Write the plan down. Export each range from the calculator and keep it with your network notes, so the next change starts from facts.
- Reserve the reverse zones. Add reverse DNS for server ranges early; mail and logging tools read those names.
Six Subnetting Mistakes That Break Networks
Each of these works on paper and fails the first busy Monday.
Overlapping ranges
A VPN cannot link two offices that share one subnet until one office renumbers.
Giving a host the .0 network address
The first address of an IPv4 range is not a host, and a device given it cannot talk properly.
Off-boundary blocks
10.0.0.32/26 is not a valid network; the block really starts at 10.0.0.0, and routing gets confused.
Mask and prefix disagreeing
A /23 typed with a 255.255.255.0 mask on one device splits the network in half without warning.
Subnets too tight
A /28 for a growing team runs out within months, forcing a disruptive change.
Public ranges used privately
Borrowing someone else’s public block internally makes their real servers unreachable from inside.
Subnet Calculator Questions People Ask
What does /24 mean?
It is the prefix length: the first 24 bits of the address are fixed, which leaves 8 bits, or 256 addresses, for the range.
How many usable addresses does a /24 give me?
There are 256 addresses, of which 254 are usable, because the first is the network and the last is the broadcast address.
What does a wildcard mask do in an ACL?
It is the inverse of the subnet mask, such as 0.0.0.255 for a /24. Access lists on many routers and firewalls use it.
Why can both addresses in a /31 be used?
RFC 3021 lets point-to-point links use both addresses, since a link between two routers needs no broadcast address.
Can the Denwix subnet calculator work with IPv6?
Yes. Type an IPv6 address with its prefix, such as 2001:db8::/48, and it works out the range, the total and the reverse zone.
Which addresses are private?
RFC 1918 keeps three blocks off the public internet for home and office use. Enter any address that falls inside 10.0.0.0/8, 172.16.0.0/12 or 192.168.0.0/16, and the Denwix calculator labels it Private (RFC 1918) in the results. In IPv6, the unique local range fc00::/7 plays the same part.
What is the reverse DNS zone for?
It is where PTR records live, which turn an address back into a name. Mail servers in particular check them.
Will the calculator accept 255.255.255.0 in place of /24?
Yes. Type the address, a space and the mask, such as 192.168.1.10 255.255.255.0, and the prefix is worked out for you.
How can I carve one block into smaller subnets?
Type a bigger prefix number into the split field. The calculator lists every smaller subnet with its first and last address.
Does any IP I enter leave my browser?
No. The calculator is plain arithmetic that runs in your browser, so nothing leaves the page.
Tools For Networks And Domains
Addressing is one part of running a site. These cover the rest.
What Is My IP
Your public address and what your browser tells every site.
Open tool →DNS Records Checker
Every record for a domain, with SPF, DMARC, DKIM and CAA checked.
Open tool →Domain Age Checker
Registration date, expiry and availability across extensions.
Open tool →
Ready To Work Out A Range?
Type an address with its prefix and the whole range appears. This IP subnet calculator costs nothing, needs no login and keeps every address in your browser.