Subnet Calculator
IP Range, Subnet Mask & Usable Host Count

What Is a Subnet Calculator and What Does It Return?

A subnet calculator takes an IPv4 address and a prefix length — 192.168.1.1 and /24, say — and works out everything a router, firewall or cloud console needs to know about that network. That means the network address, the broadcast address, the subnet mask, the first and last usable host, the total address count and the wildcard mask. The tool above returns all of it, plus the conventional default gateway, which most calculators leave out.

The same tool answers to several names. Subnet mask calculator, netmask calculator, IP subnet calculator, IP range calculator, IP address calculator and subnetting calculator all describe one job: turning an address plus a mask into a complete, usable address plan. Whichever term you searched for, this page does it, for every prefix from /1 to /32.

You will also see it written as one word — subnetmask — or shortened to submask in older documentation and exam material. Those refer to the same 32-bit value. Everything here runs online in the browser with no account and nothing to install, so calculating a netmask takes about as long as typing the address.

What each output actually means

• Network address — the first address in the block. It names the subnet and cannot be given to a device.
• Broadcast address — the last address. Traffic sent here reaches every host at once, so it is also unassignable.
• Usable host range — everything between those two. This is what you hand to devices or a DHCP scope.
• Subnet mask — the 32-bit value that marks where the network part ends and the host part begins.
• Wildcard mask — the mask inverted. Cisco access lists and OSPF statements take this form rather than the mask.
• IP class and scope — whether the block is public, private under RFC 1918, carrier-NAT or reserved.

Subnet Mask Reference: Every Prefix From /8 to /32

This is the lookup table rather than the calculator — useful when you already know the prefix and just want the mask, the host count, or where the next block starts. Every prefix from /8 to /32 is listed with its subnet mask, total addresses, usable hosts, and the step between consecutive blocks. The four rows people ask about most — /23, /25, /27 and /29 — are highlighted.

Reading the block step column

The step tells you where the next block of the same size begins, and which octet moves. A /22 steps by 4 in the third octet, so valid blocks start at 10.0.0.0, 10.0.4.0, 10.0.8.0 and so on — 10.0.5.0/22 is not a legal block start, and most routers will quietly widen it to 10.0.4.0/22 rather than warning you.

Subnet Mask to Prefix Length, and Back Again

The prefix length is simply the count of 1 bits in the subnet mask. 255.255.255.0 is twenty-four 1 bits, so it is /24. Going the other way, write the prefix as that many 1 bits and pad with zeros. The two notations carry identical information — they are the same value written differently.

Knowing which one a system wants matters more than it should. Windows network settings and macOS both label the field prefix length, Linux tools accept either, and most router interfaces still ask for a dotted netmask. Converting between them by hand is where the mistakes creep in, especially for masks that do not end on an octet boundary.

The nine values a mask octet can hold

A legal mask is an unbroken run of 1s followed by 0s, which leaves exactly nine possible values for the octet where the boundary falls. Everything else in the mask is 255 or 0. Learn these nine and you can convert any mask on sight — 255.0.255.0 looks plausible but is invalid, because the run of 1s is broken.

The same nine values apply in any octet. 255.255.254.0 puts the 254 in the third octet — seven 1 bits there on top of sixteen before it, so twenty-three in total, which is a /23. The full mask-to-prefix mapping for all 25 sizes is in the reference table above, and the tool returns both notations for whatever you enter.

What Is the Default Gateway and How Do You Choose It?

Here is the honest answer nobody seems to give: the default gateway is not calculated. It is a choice. It is whichever address you assign to the router interface that carries traffic out of the subnet, and any usable address in the range will work. What exists is a strong convention, not a formula.

The convention

• First usable address — 10.0.1.1 in a 10.0.1.0/24. This is what almost every home router, ISP and cloud platform uses, and it is what the tool above shows.
• Last usable address — 10.0.1.254 in the same block. Common in enterprise networks that reserve the low end for infrastructure. The tool shows this too, so you have both.
• Anything in between — legal, works fine, and will confuse the next person who touches the network. Pick one convention and hold to it site-wide.

Two rules are genuinely fixed rather than conventional. The gateway must sit inside the same subnet as the hosts using it — a device on 10.0.1.0/24 cannot use 10.0.2.1 as its gateway, because it would have to route through the gateway to reach the gateway. And it can never be the network or broadcast address, which is why the calculator offers only usable addresses.

VLSM: Using Different Subnet Sizes in One Network

VLSM, or variable length subnet masking, means giving each subnet the size it actually needs instead of cutting everything to one mask. The older approach, FLSM, uses a single mask throughout — simpler to plan, and wasteful the moment your segments differ in size.

The difference shows up fast. Take a 192.168.1.0/24 that has to serve 100 workstations, 50 phones, 20 servers and three router links. Split it into four equal /26 blocks and the router links get 62 addresses each to use two, while the workstation segment runs out at 62 of the 100 it needs. Size each one instead and it fits comfortably:

Allocating largest first is the part people skip, and it is what keeps the plan working. Start with the small blocks and the big one has nowhere aligned to land. Every subnet still has to begin on a boundary that matches its own size, which the reference table above gives you at a glance.

The reverse operation is supernetting — merging adjacent blocks into one larger route rather than splitting a block into smaller ones. It follows the same alignment rules read backwards, and our guide to route aggregation and CIDR blocks covers it properly, including turning a start-and-end range back into the blocks that cover it.

What Is the Wildcard Mask in Your Results?

The wildcard mask — also called the inverse mask, and both names appear in Cisco documentation — is the subnet mask flipped: where the mask has a 1, the wildcard has a 0. A /24 mask of 255.255.255.0 becomes a wildcard of 0.0.0.255. The tool returns it alongside the mask because Cisco access lists and OSPF network statements take the wildcard form, not the mask, and you need it the moment you touch IOS.

• /24 mask 255.255.255.0 → wildcard 0.0.0.255
• /26 mask 255.255.255.192 → wildcard 0.0.0.63
• /16 mask 255.255.0.0 → wildcard 0.0.255.255

Pasting a subnet mask where a wildcard belongs is one of the most common lab mistakes there is, and nothing warns you — both values are valid syntax, so the ACL loads and then matches an address set you never intended. The full ACL and OSPF configuration guide, with worked examples, lives on our CIDR and wildcard mask reference rather than being repeated here.