What does this subnet calculator do?

Enter any IPv4 or IPv6 address with a CIDR prefix and it returns the network address, broadcast address, subnet mask, wildcard mask, usable host range, host count, binary breakdown and address classification — instantly, in your browser. It also plans variable-length subnets, and includes a full CIDR reference table.

Do I have to type the mask as well as the prefix?

No — either one is enough. Type 192.168.1.10/24 and the mask is derived; type the dotted mask 255.255.255.0 and the prefix is derived. The calculator accepts a host address as well as a network address and works out which network it belongs to.

What are the four tabs for?

• Calculator — full detail for one IPv4 subnet • VLSM — split a block into subnets sized to real host requirements • Cheat sheet — every prefix from /0 to /32 with masks and host counts • IPv6 — expand, compress and calculate IPv6 prefixes

Can I link straight to a subnet?

Yes, and the address bar stays in sync as you type. ?ip=192.168.1.0&cidr=24 loads that subnet; the CIDR can be inline (?ip=10.0.0.0/8), you can pass ?mask=255.255.255.0 instead, open a tab with ?tab=calc|vlsm|cheat|ipv6, and link an IPv6 prefix with ?tab=ipv6&ip6=2001:db8::&prefix6=64.

Is anything sent to a server?

No. Every calculation is arithmetic done in JavaScript in your tab. Nothing is uploaded and nothing is stored, which matters when the addresses describe a customer's internal network.

Does it work offline?

Yes, once the page has loaded. There are no external libraries and no API calls, so it keeps working on a plane, in a datacentre aisle or on a locked-down jump box.

Free, no account, no limits, no ads. One of a set of client-side tools at jasperbernaers.com/apps.

What is CIDR notation?

Classless Inter-Domain Routing notation writes an address followed by the number of network bits: 192.168.1.0/24 means the first 24 bits identify the network and the remaining 8 identify hosts. It replaced classful addressing in 1993 (RFC 1519) because Class A, B and C blocks wasted enormous amounts of address space.

How do I convert a CIDR prefix to a subnet mask?

Write the prefix as that many 1 bits followed by 0s, then group into four octets. /24 → 11111111.11111111.11111111.00000000 → 255.255.255.0. /20 → 255.255.240.0. The only octet values you will ever see in a mask are 0, 128, 192, 224, 240, 248, 252, 254 and 255.

How do I convert a subnet mask back to a prefix?

Count the 1 bits. 255.255.255.192 is 8+8+8+2 = /26. A quick shortcut for the last octet: 128→1, 192→2, 224→3, 240→4, 248→5, 252→6, 254→7, 255→8.

What is a wildcard mask?

The bitwise inverse of the subnet mask. For 255.255.255.0 the wildcard is 0.0.0.255. Cisco ACLs, OSPF network statements and several other IOS commands take a wildcard rather than a mask — a 0 bit means “must match”, a 1 bit means “don't care”.

Why is the mask sometimes written /24 and sometimes 255.255.255.0?

They are the same thing in different notations. Linux, cloud providers, routing protocols and modern documentation use CIDR; Windows dialogs, older Cisco interface configuration and many appliances use the dotted mask. Wildcard masks are a third form of the same information.

Can a subnet mask have non-contiguous bits?

Not in practice. A valid mask is an unbroken run of 1s followed by 0s. Something like 255.255.0.255 is a discontiguous mask — historically legal in a few implementations, rejected by essentially everything modern, and a reliable sign of a typo.

A two-address block with no network or broadcast address, defined by RFC 3021 for point-to-point links. Both addresses are usable, which halves the waste compared with a /30. Most modern routers support it; some older gear does not, which is why /30 is still common on WAN links.

A single address — a host route. You see it on loopback interfaces, in firewall rules that match exactly one host, in BGP advertisements of a single address, and in VPN split-tunnel configuration.

The whole IPv4 address space, 0.0.0.0/0 — the default route. In a firewall rule it means “any source” or “any destination”, which is why 0.0.0.0/0 on an inbound rule deserves a second look.

The opposite of subnetting: combining adjacent blocks into one shorter prefix. Four contiguous /24s aligned on a boundary become a single /22, which is how route summarisation shrinks routing tables. The blocks must be contiguous and correctly aligned — 192.168.4.0/24 through 192.168.7.0/24 summarise to 192.168.4.0/22, but starting at .5 does not.

Why can't I use any starting address for a subnet?

Because a subnet must start on a boundary that is a multiple of its own size. A /26 (64 addresses) can start at .0, .64, .128 or .192 — never at .50. If a tool tells you your network address is not what you typed, this alignment rule is why.

How many usable hosts does a subnet have?

2^(32 − prefix) − 2 for IPv4. A /24 has 2⁸ = 256 addresses, minus the network and broadcast addresses, so 254 usable. A /26 has 62, a /28 has 14, a /30 has 2. The exception is /31, where both addresses are usable, and /32, which is a single host.

The first address in the block has all host bits set to 0 and identifies the network itself; the last has all host bits set to 1 and is the directed broadcast address. Neither can be assigned to an interface, so every subnet costs two addresses in overhead.

What is the network address?

The result of ANDing the IP address with the subnet mask — the first address in the range. 192.168.1.77/26 AND 255.255.255.192 gives 192.168.1.64. It is what appears in a routing table.

What is the broadcast address?

The last address in the subnet: the network address with every host bit set to 1. For 192.168.1.64/26 it is 192.168.1.127. Traffic sent there reaches every host on the local segment — which is also why very large subnets are a bad idea.

Which address is the gateway?

Nothing in the standard says. By convention it is the first usable address (.1 in a /24) or sometimes the last (.254). Cloud providers make their own choice — AWS and Azure both reserve the first usable address in every subnet for the router.

How many addresses does a /22 have?

1,024 total and 1,022 usable — the equivalent of four /24s. Each step down the prefix doubles the block: /23 is 512, /22 is 1,024, /21 is 2,048, /20 is 4,096, /16 is 65,536.

What is the “magic number” trick for subnetting in your head?

Subtract the interesting octet of the mask from 256. For /26 the mask octet is 192, so 256 − 192 = 64: subnets start at .0, .64, .128 and .192. For /28 the step is 16, for /27 it is 32. Find the block your host falls into and you have the network and broadcast addresses without any binary.

How do I find which subnet an address belongs to?

Round the address down to the nearest multiple of the block size. 10.1.2.200/27 has a block size of 32, and 200 ÷ 32 = 6 remainder 8, so the network is 10.1.2.192, the broadcast is 10.1.2.223, and the usable range is .193–.222. Typing the host address into the calculator does exactly this.

Are two addresses in the same subnet?

Apply the same mask to both and compare the results. 10.0.0.5/24 and 10.0.1.5/24 give networks 10.0.0.0 and 10.0.1.0 — different subnets, so they need a router between them. This is the calculation behind almost every “I can't reach it” ticket.

What is the difference between a subnet and a VLAN?

A VLAN is a layer-2 broadcast domain; a subnet is a layer-3 address range. They are usually mapped one-to-one, so people use the words interchangeably, but they are different layers — you can technically put two subnets on one VLAN (secondary addressing), and it is almost always a mistake.

Variable Length Subnet Masking means using different prefix lengths inside one parent block instead of splitting it into equal parts. A /24 can hold a /25 for 100 users, a /27 for 20 servers, a /28 for printers and a /30 for a WAN link — sized to what each segment actually needs.

How do I plan subnets with VLSM?

List every segment with its host count, sort largest first, then allocate from the top of the block downwards. Each subnet must start on a boundary that is a multiple of its own size, and allocating big-to-small keeps that alignment automatic. The VLSM tab does the arithmetic and shows what is left over.

Why does VLSM allocation have to start with the biggest subnet?

Because of alignment. If you place a /28 first at .0 and then need a /25, the /25 cannot start until .128, and you have stranded 112 addresses. Largest first packs the block without holes.

How much headroom should I leave in a subnet?

Size for roughly twice the current host count, then round up to the next prefix. A segment with 50 devices fits in a /26 (62 usable) but has almost no room; a /25 (126 usable) survives growth. Resizing a subnet in production means re-addressing every host on it.

Is it better to use one big subnet or several small ones?

Several. A single flat /16 puts 65,000 hosts in one broadcast domain — ARP and broadcast traffic scale badly, and any host can reach any other with nothing in between. Smaller subnets give you segmentation points for firewalls and ACLs.

How should I structure addressing across sites?

Allocate a contiguous block per site so it can be summarised into one route — for example a /20 per site, subdivided internally. Encoding meaning into the octets (site, function, VLAN) makes ACLs shorter and routing tables smaller. The design that cannot be summarised is the one you regret at 200 sites.

Should I use 10.0.0.0/8 or 192.168.0.0/16 for a company network?

Use 10.0.0.0/8 for anything that might grow, because it gives you room to encode site and function and to summarise. 192.168.0.0/16 is fine for a home or a single small office, but it is also what every consumer router uses, which guarantees VPN conflicts with home workers.

Why do overlapping ranges cause VPN problems?

If both ends of a tunnel use 192.168.1.0/24, a host cannot tell whether a destination is local or remote — the local route always wins and the traffic never enters the tunnel. Picking an unusual range from 10.0.0.0/8 for the office avoids the collision entirely; the fix afterwards is NAT, which nobody enjoys.

How big should a point-to-point link subnet be?

A /31 if your equipment supports RFC 3021, otherwise a /30. Using a /24 for a link with two routers on it wastes 252 addresses, and it happens more often than it should.

What are the private IPv4 ranges?

RFC 1918 reserves three blocks: 10.0.0.0/8 (16,777,216 addresses), 172.16.0.0/12 (172.16.0.0–172.31.255.255, about a million), and 192.168.0.0/16 (65,536). They are never routed on the public internet, so every organisation can use them internally.

Is 172.16.0.0/12 the same as 172.16.0.0/16?

No, and this is a classic mistake. The private block is a /12, covering 172.16 through 172.31. A /16 would only cover 172.16.x.x. Firewall rules written with the wrong prefix here silently miss most of the range.

What is 127.0.0.0/8 for?

Loopback. The whole /8 belongs to the local host, not just 127.0.0.1 — you can bind services to 127.0.0.2 or 127.1.2.3 and they never leave the machine.

What is 169.254.0.0/16?

Link-local, also called APIPA. A host assigns itself one of these when DHCP fails, so seeing a 169.254 address on a client almost always means “no DHCP server answered”. Cloud instance metadata services also live here, at 169.254.169.254.

What is 100.64.0.0/10?

Carrier-grade NAT space (RFC 6598), used by ISPs to number customer equipment when they have run out of public addresses. It is not routable on the internet and not RFC 1918 — if you see it on a home connection, you are behind CGNAT, which is why inbound port forwarding will not work.

Which ranges are reserved for documentation?

192.0.2.0/24, 198.51.100.0/24 and 203.0.113.0/24 (RFC 5737), plus 2001:db8::/32 for IPv6. Use these in examples, diagrams and blog posts rather than someone's real address space.

Multicast. Inside it, 224.0.0.0/24 is link-local control traffic — 224.0.0.5 and .6 for OSPF, 224.0.0.9 for RIPv2, 224.0.0.251 for mDNS. 240.0.0.0/4 above it is reserved and effectively unusable.

Are Class A, B and C still relevant?

Only as vocabulary. Classful addressing was replaced by CIDR in 1993, and no modern router assumes a mask from the first octet. People still say “a class C” when they mean a /24 — harmless as shorthand, misleading if you actually believe the mask is implied.

Can I use public addresses internally without owning them?

Technically yes, and it is a well-known way to create a problem for yourself: the day someone needs to reach the real owner of that block, they cannot. Squatting on 1.1.1.0/24 or on another company's space breaks exactly when you least want it to.

How many addresses does AWS reserve in a subnet?

Five per subnet: the network address, the VPC router (.1), the DNS resolver (.2), one reserved for future use (.3), and the broadcast address. A /24 in AWS gives you 251 usable addresses, not 254 — and a /28, the smallest AWS allows, gives you 11.

What are the subnet size limits in AWS?

A VPC CIDR can be /16 to /28, and subnets follow the same range. Anything smaller than a /28 is rejected, which is why a two-instance subnet still costs 16 addresses.

How many addresses does Azure reserve?

Five as well: the network address, the default gateway (.1), two for Azure DNS (.2 and .3), and the broadcast address. Azure subnets can be /2 to /29, so a /29 leaves three usable addresses.

GCP reserves four addresses per subnet: the network address, the default gateway, the second-to-last address, and the broadcast address. Subnet ranges are also expandable in place, which is unusual and very useful.

Why does my cloud subnet run out of addresses so fast?

Because managed services consume them invisibly. Load balancer nodes, NAT gateways, private endpoints, container ENIs and database instances all take addresses from your subnet. Kubernetes with AWS VPC CNI assigns a real VPC address to every pod, so a /24 node subnet can be exhausted by a few dozen pods.

Free Subnet Calculator — IPv4/IPv6 CIDR, VLSM, Wildcard Mask & Network Planner Online

IP Address

CIDR Prefix

Prefix Length /24

VLSM (Variable Length Subnet Masking) divides a network into subnets of different sizes. Enter a network and add subnets with their required host counts — the planner allocates the most efficient CIDR blocks automatically.

Network Address

CIDR

▸ Subnets Required

Subnet Name

Hosts Needed

Complete IPv4 CIDR cheat sheet — all 33 prefix lengths from /0 to /32 with subnet masks, wildcard masks, total addresses, and usable hosts.

IPv6 Subnet Calculator — enter an IPv6 address with prefix length to see the full network range, total addresses, and expanded notation.

IPv6 Address

Prefix

Free Online Subnet Calculator — IPv4/IPv6 CIDR, VLSM & Network Planning

This free subnet calculator instantly computes all subnet details from any IPv4 or IPv6 address with CIDR prefix. Whether you're a network engineer, sysadmin, CCNA student, or cloud architect, get instant answers for subnet masks, wildcard masks, network and broadcast addresses, usable host ranges, and binary breakdowns. All calculations happen instantly in your browser — no data is sent to a server, no signup required.

Subnet Calculator Features

▸ IPv4 Subnet Calculator — enter any IP address with CIDR prefix (/0 to /32) for instant network address, broadcast, mask, and host range
▸ VLSM Planner — divide a network into variable-length subnets sized to exact host requirements, minimising address waste
▸ CIDR Cheat Sheet — complete reference table of all 33 prefix lengths (/0–/32) with subnet masks, wildcard masks, and usable host counts
▸ IPv6 Calculator — expand compressed IPv6 addresses, calculate network prefix, first/last address, and total address count for any prefix length up to /128
▸ Binary Breakdown — visual binary representation of IP address, subnet mask, and network address, colour-coded by network vs host bits
▸ IP Classification — automatic detection of IP class (A/B/C/D/E), private vs public (RFC 1918), loopback, APIPA, CGNAT, multicast, and reserved ranges

Common Subnet Reference — Most Searched CIDR Blocks

These are the subnets network engineers look up most often. Each row shows the subnet mask, wildcard mask, usable host count, and a typical use case:

CIDR	Subnet Mask	Wildcard	Usable Hosts	Typical Use
/8	255.0.0.0	0.255.255.255	16,777,214	Class A networks (10.0.0.0/8)
/16	255.255.0.0	0.0.255.255	65,534	Large campus / VPC supernets
/24	255.255.255.0	0.0.0.255	254	Standard office / home LAN (192.168.1.0/24)
/25	255.255.255.128	0.0.0.127	126	Split /24 into two equal halves
/26	255.255.255.192	0.0.0.63	62	Small department / VLAN
/27	255.255.255.224	0.0.0.31	30	Server segments / DMZ
/28	255.255.255.240	0.0.0.15	14	Small server cluster / management VLAN
/29	255.255.255.248	0.0.0.7	6	Small server group
/30	255.255.255.252	0.0.0.3	2	Point-to-point WAN links / router interfaces
/32	255.255.255.255	0.0.0.0	1	Host routes, loopback interfaces, firewall rules

RFC 1918 Private Address Ranges

These address blocks are reserved for private networks and are not routed on the public internet. Use them for internal LANs, VPCs, and VLANs:

Range	Class	Total Addresses	Typical Use
10.0.0.0/8	A	16,777,216	Large enterprise networks, cloud VPCs (AWS, Azure, GCP)
172.16.0.0/12	B	1,048,576	Medium-sized networks, Docker default bridge (172.17.0.0/16)
192.168.0.0/16	C	65,536	Home & small office networks (192.168.1.0/24 is the most common LAN)

Who Uses a Subnet Calculator?

Network engineers, system administrators, DevOps teams, cloud architects, CCNA/CCNP students, and anyone working with IP addressing and subnetting. Common use cases include:

▸ Planning office and campus networks with appropriate VLAN segmentation
▸ Configuring VPC subnets in AWS, Azure, or Google Cloud with the right CIDR blocks
▸ Writing Cisco ACLs and OSPF configurations that require wildcard masks
▸ Designing VLSM schemes to maximise IP address efficiency across branch offices
▸ Studying for Cisco CCNA, CCNP, CompTIA Network+, and similar networking certifications
▸ Configuring firewall rules and security groups with precise CIDR-based source/destination filters
▸ Verifying subnetting homework and exam answers with instant binary breakdown

How to Read Subnet Calculator Results

When you enter 192.168.1.100/24, the subnet calculator shows: Network Address 192.168.1.0 (the identifier for the subnet), Broadcast Address 192.168.1.255 (sends to all hosts), Subnet Mask 255.255.255.0 (used in device configuration), Wildcard Mask 0.0.0.255 (used in ACLs and routing protocols), First Usable Host 192.168.1.1, Last Usable Host 192.168.1.254, and 254 usable hosts out of 256 total addresses. The binary breakdown visually separates network bits (first 24) from host bits (last 8), making it clear which portion of the address identifies the network versus the device.

// Subnet Calculator & Network Planner_

Subnetting, CIDR and IP planning — every question answered

01. Using this calculator

02. CIDR, prefixes and subnet masks

03. Hosts, network and broadcast addresses

04. VLSM and network design

05. Private, reserved and special ranges

06. Cloud subnets and containers

07. IPv6

08. Troubleshooting and common mistakes

Free Online Subnet Calculator — IPv4/IPv6 CIDR, VLSM & Network Planning

Subnet Calculator Features

Common Subnet Reference — Most Searched CIDR Blocks

RFC 1918 Private Address Ranges

Who Uses a Subnet Calculator?

How to Read Subnet Calculator Results

🌐 Language

// Subnet Calculator & Network Planner_

Subnetting, CIDR and IP planning — every question answered

01. Using this calculator

02. CIDR, prefixes and subnet masks

03. Hosts, network and broadcast addresses

04. VLSM and network design

05. Private, reserved and special ranges

06. Cloud subnets and containers

07. IPv6

08. Troubleshooting and common mistakes

Free Online Subnet Calculator — IPv4/IPv6 CIDR, VLSM & Network Planning

Subnet Calculator Features

Common Subnet Reference — Most Searched CIDR Blocks

RFC 1918 Private Address Ranges

Who Uses a Subnet Calculator?

How to Read Subnet Calculator Results