IPv4 Subnet Calculator

What Is a Subnet Mask?

// Subnetting is integer arithmetic on a 32-bit value.
const toInt = (ip) => ip.split('.').reduce((acc, o) => acc * 256 + Number(o), 0);
const toIp  = (n)  => [24, 16, 8, 0].map((s) => (n >>> s) & 255).join('.');

function subnet(ip, prefix) {
  const mask      = prefix === 0 ? 0 : (0xFFFFFFFF << (32 - prefix)) >>> 0;
  const network   = (toInt(ip) & mask) >>> 0;
  const broadcast = (network | (~mask >>> 0)) >>> 0;
  // RFC 3021: a /31 has two usable addresses; a /32 has one.
  const usable    = prefix >= 31 ? 2 ** (32 - prefix) : 2 ** (32 - prefix) - 2;
  return { network: toIp(network), broadcast: toIp(broadcast), usable };
}

subnet('192.168.1.130', 26);
// { network: '192.168.1.128', broadcast: '192.168.1.191', usable: 62 }

Key Features

Subnet Calculation Examples

192.168.1.130/26

Network:   192.168.1.128
Broadcast: 192.168.1.191
Usable:    192.168.1.129 - 192.168.1.190
Hosts:     62 usable of 64 total
Netmask:   255.255.255.192
Wildcard:  0.0.0.63

10.0.0.0/22

Network:   10.0.0.0
Broadcast: 10.0.3.255
Usable:    10.0.0.1 - 10.0.3.254
Hosts:     1,022 usable of 1,024 total
Netmask:   255.255.252.0
Wildcard:  0.0.3.255

203.0.113.4/31

Addresses: 203.0.113.4 and 203.0.113.5
Usable:    2 (both ends of the link)
Broadcast: none
Netmask:   255.255.255.254
Wildcard:  0.0.0.1

192.168.1.0/24 → /26

192.168.1.0/26     192.168.1.1 - 192.168.1.62     bc 192.168.1.63    62 hosts
192.168.1.64/26    192.168.1.65 - 192.168.1.126    bc 192.168.1.127   62 hosts
192.168.1.128/26   192.168.1.129 - 192.168.1.190   bc 192.168.1.191   62 hosts
192.168.1.192/26   192.168.1.193 - 192.168.1.254   bc 192.168.1.255   62 hosts

/24, /25, /26, /27, /28, /29, /30, /31, /32

/24  255.255.255.0     256 addresses   254 usable
/25  255.255.255.128   128 addresses   126 usable
/26  255.255.255.192    64 addresses    62 usable
/27  255.255.255.224    32 addresses    30 usable
/28  255.255.255.240    16 addresses    14 usable
/29  255.255.255.248     8 addresses     6 usable
/30  255.255.255.252     4 addresses     2 usable
/31  255.255.255.254     2 addresses     2 usable (RFC 3021)
/32  255.255.255.255     1 address       1 usable (host route)

How to Use the IPv4 Subnet Calculator

1. 1

   Enter an address or CIDR block

   Type 192.168.1.130/26, paste 10.0.0.0 255.255.252.0, or enter a bare address and set the prefix with the slider. A host address is fine — the tool masks it down to its network for you.

2. 2

   Move the prefix slider to see the boundary shift

   Drag from /0 to /32 and watch the host count, mask and broadcast address recalculate. The slider keeps your address fixed, so it answers 'what if I resize this block?' in one gesture.

3. 3

   Read the subnet details

   Network address, broadcast, first and last usable host, usable and total counts, subnet mask and wildcard mask — plus a badge naming the block (private, CGNAT, link-local, documentation, multicast) and its historical class.

4. 4

   Check the binary view and address forms

   Network bits and host bits are coloured separately so the mask boundary is visible bit by bit. The same panel gives the network as an integer, as hexadecimal, and as a reverse-DNS in-addr.arpa name.

5. 5

   Test membership and divide the block

   Paste any address to confirm whether it falls inside the current subnet, then pick a longer prefix to split the block into equal child subnets with their ranges, broadcasts and host counts.

Common Subnetting Mistakes

192.168.1.130/26 -> network 192.168.1.0, broadcast 192.168.1.255

192.168.1.130/26 -> network 192.168.1.128, broadcast 192.168.1.191

203.0.113.4/31 -> 0 usable hosts

203.0.113.4/31 -> 2 usable hosts (203.0.113.4 and 203.0.113.5)

172.20.5.1 -> assumed public because it is not 172.16.x.x

172.20.5.1 -> private, inside 172.16.0.0/12 (172.16.0.0 - 172.31.255.255)

10.0.0.0 255.0.255.0

10.0.0.0 255.255.252.0   (= 10.0.0.0/22)

What You Can Do with the Subnet Calculator

Plan VLANs and address blocks

Divide an allocation into equal subnets, confirm each one is large enough for its host count, and copy the layout straight into your IPAM spreadsheet or design document before it reaches a switch.

Write firewall rules and ACLs

Read the wildcard mask for Cisco ACLs and OSPF network statements, and confirm the exact address range a rule will match — the difference between 0.0.0.63 and 0.0.0.31 is two VLANs' worth of hosts.

Troubleshoot a host that cannot reach the gateway

Paste the host address and the mask it was configured with, then check whether the gateway falls inside the resulting subnet. A mismatched mask is the classic cause of 'it pings some things but not others'.

Number router-to-router links efficiently

Compare /30 against /31 for point-to-point links: the /31 gives both endpoints an address instead of wasting two per link, which adds up quickly across a WAN with hundreds of circuits.

Study for CCNA, Network+ and similar exams

Work a subnetting question by hand, then check the network address, broadcast and host range here. The binary view shows the borrowed bits, which is what the exam is actually testing.

How IPv4 Subnetting Works

The mask is a bit boundary, not a decimal value

A subnet mask is 32 bits: a contiguous run of 1s for the network, then 0s for hosts. Only 33 masks are valid (/0 through /32), which is why 255.255.255.192 is legal and 255.0.255.0 is not — the latter has a hole in it. This calculator rejects non-contiguous masks rather than guessing what you meant.

Block size and the fourth-octet shortcut

Subnets of the same size sit on multiples of their block size, and the block size is 256 minus the relevant mask octet. A /26 ends in 192, so blocks start at 0, 64, 128 and 192; a /28 ends in 240, so they step every 16. This is the arithmetic behind 'which subnet does 192.168.1.130 belong to?' — it is the third /26 block, hence 192.168.1.128.

Reserved addresses and the two exceptions

The all-zeros host address identifies the network and the all-ones host address is the directed broadcast, so ordinary subnets lose two addresses. RFC 3021 waives both for /31 point-to-point links (two usable addresses), and a /32 is a single host route (one usable address). The reference table under the calculator follows those rules, so its /31 row reads 2 rather than 0.

Wildcard masks and where they are required

The wildcard mask is the bitwise inverse of the subnet mask — 255.255.255.192 inverts to 0.0.0.63. Cisco ACLs, OSPF network statements and EIGRP take wildcards instead of masks. Unlike subnet masks, ACL wildcards may be non-contiguous, which is how a single rule can match every odd-numbered address in a range.

Classes are history, blocks are current

Class A/B/C/D/E divides the address space by first octet (0–127, 128–191, 192–223, 224–239, 240–255) and is still taught and examined, but CIDR replaced classful routing in 1993 — the mask decides where a network ends, not the leading bits. What does still matter is which reserved block an address falls into: RFC 1918 private, RFC 6598 CGNAT, RFC 3927 link-local, RFC 5737 documentation, RFC 2544 benchmarking, multicast and the 240.0.0.0/4 reserved range.

IP Addressing Best Practices

Size the subnet for growth, but not by a factor of ten

A /24 for eight devices wastes 246 addresses and enlarges the broadcast domain for no benefit. Pick the smallest prefix that leaves comfortable headroom — the calculator's host count makes the trade-off explicit before you commit.

Use /31 on point-to-point links

On router-to-router circuits a /31 gives both ends an address instead of burning four addresses for two endpoints. Reserve /30 for equipment that genuinely predates RFC 3021 support, and never put a /31 on a multi-access segment.

Allocate on a bit boundary, not on round decimal numbers

Address plans that start blocks at 10.0.10.0 or 10.0.100.0 look tidy in decimal but cannot be summarised into a single route. Align allocations to powers of two so a whole region collapses into one prefix in the routing table.

Keep documentation examples in the reserved ranges

RFC 5737 reserves 192.0.2.0/24, 198.51.100.0/24 and 203.0.113.0/24 precisely so that examples cannot collide with a real network. Using a random public address in a runbook eventually points someone at a live host.

Verify the mask on both ends before blaming routing

When a host reaches some destinations but not others, compare its mask against the gateway's. A /24 configured where the segment is a /23 makes half the network look remote — paste both addresses into the membership check and the mismatch shows up immediately.

Subnet Calculator FAQ