CIDR Calculator — Subnet, IP Range & Network Address Tool
Enter any IP address with a CIDR prefix to instantly get your network address, broadcast address, usable host range, subnet mask, and wildcard mask. No manual binary math — just results.
✓Verified: RFC 4632 CIDR standard & RFC 950 subnetting procedure — April 2026
Enter any IP address with its CIDR prefix. We handle both the calculation and the explanation.
IPv4 address (e.g. 10.0.0.1 or 192.168.1.50)Enter a valid IPv4 address.
Network Address
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⚠️ Disclaimer: Results follow RFC 4632 CIDR and RFC 950 subnetting standards. Usable hosts = 2^(32−prefix)−2 except /31 (RFC 3021, 2 usable) and /32 (1 host). AWS VPCs reserve 5 IPs per subnet instead of 2.
Have an IP in CIDR format like 10.0.0.0/24? Enter it directly to find the full range in one step.
Enter as IP/prefix — e.g. 10.0.0.0/8 or 172.16.0.0/12Enter a valid IP/prefix format (e.g. 192.168.1.0/24).
Network / CIDR
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The complete CIDR cheat sheet. Every prefix from /0 to /32 with IPs, usable hosts, subnet mask, and common use case.
Sources & Methodology
✓All CIDR calculations follow RFC 4632 (Classless Inter-Domain Routing) and RFC 950 (Internet Standard Subnetting Procedure). Formula: Network = IP AND Mask. Broadcast = Network OR Wildcard. Usable = 2^(32−prefix)−2.
The definitive IETF standard defining CIDR notation, variable-length prefix routing, and the aggregation model that replaced classful A/B/C addressing. Primary reference for all prefix-length calculations in this calculator.
Foundational RFC defining subnet mask usage, the bitwise AND operation for network address derivation, and broadcast address calculation. Source for the core subnetting formulas applied by this calculator.
Defines the three private IPv4 address ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) identified and labeled in the calculator results. The RFC 1918 classification displayed alongside each calculation result.
CIDR Calculator Guide — From Confusion to Clarity
If you've ever tried to calculate a subnet by hand — converting IPs to binary, ANDing masks, counting bits — you know how tedious it gets. And it's not because subnetting is hard. It's because the manual process is designed for computers, not humans. CIDR notation was invented to make IP addressing more flexible and efficient, but the notation itself trips up engineers at every level.
Most people overcomplicate CIDR. In reality, it's just a compact way to say how big a network is. Once that clicks, everything else follows naturally.
What Is CIDR? (The Simple Version)
CIDR stands for Classless Inter-Domain Routing. It was introduced in 1993 to replace the old class-based system where every IPv4 network was either a Class A (/8), Class B (/16), or Class C (/24). That system was so wasteful that we would have exhausted IPv4 addresses by the late 1990s without it.
The format is simple: an IP address followed by a slash and a number. That number — the prefix length — tells you how many bits of the address identify the network. The rest belong to individual hosts.
192.168.1.0/24 — the first 24 bits are the network, 8 bits are for hosts
10.0.0.0/8 — the first 8 bits are the network, 24 bits are for hosts
172.16.5.0/30 — the first 30 bits are network, only 2 bits for hosts
That number after the slash is everything. Bigger number = smaller network = fewer hosts. Smaller number = larger network = more hosts. That's the whole mental model.
The CIDR Formula — How the Math Actually Works
There are three numbers you'll calculate from any IP/prefix combination. Once you understand these, you can work with any CIDR block without a calculator — though the calculator obviously saves time.
Total IPs = 2^(32 − prefix_length)Usable Hosts = Total IPs − 2 (network address + broadcast reserved)Subnet Mask = 32-bit number with prefix_length leading 1s, rest 0sNetwork Address = IP AND Subnet Mask (bitwise)Broadcast = Network OR Wildcard Mask (wildcard = NOT subnet mask)
Quick examples:
/24 → 2^8 = 256 total IPs → 254 usable
/16 → 2^16 = 65,536 total IPs → 65,534 usable
/30 → 2^2 = 4 total IPs → 2 usable
/32 → 2^0 = 1 total IP → single host address (loopback, etc.)
Step-by-Step: How to Calculate 192.168.1.0/24
Let's walk through 192.168.1.0/24 manually, the exact same way the calculator does it. This is the most common subnet you'll ever encounter — practically every home and office router uses a /24.
1
Convert the prefix to a subnet mask
/24 means 24 ones followed by 8 zeros: 11111111.11111111.11111111.00000000 In dotted decimal: 255.255.255.0
2
Find the network address (IP AND mask)
192.168.1.0 AND 255.255.255.0 = 192.168.1.0 The host octet (0) AND 0 = 0, so it stays .0
3
Calculate the wildcard mask (invert the subnet mask)
255.255.255.255 − 255.255.255.0 = 0.0.0.255
4
Find the broadcast address (network OR wildcard)
192.168.1.0 OR 0.0.0.255 = 192.168.1.255
5
Identify the usable range
First usable: network address + 1 = 192.168.1.1 Last usable: broadcast − 1 = 192.168.1.254 Total usable: 256 − 2 = 254 hosts
Real-World Examples: /24, /16, and /30
Different prefix lengths solve different problems. Here are the three you'll encounter most often in real deployments.
/24 — The Home and Office Standard. 192.168.1.0/24 is in practically every home router on the planet. 254 usable hosts is enough for any house or small office, and the math is clean: the first three octets identify the network, the last one identifies the host. Network: 192.168.1.0. Broadcast: 192.168.1.255. Range: .1 to .254. Simple.
/16 — When You Need Room to Grow. 10.0.0.0/16 gives you 65,534 usable addresses across the 10.0.0.0 through 10.0.255.255 range. You'll see /16 in corporate data centers, AWS default VPCs (172.31.0.0/16), and anywhere with multiple departments needing their own /24 subnets carved out of a larger space. Broadcast: 10.0.255.255.
/30 — The Router Link Special. 10.0.0.0/30 has only 4 IPs — 10.0.0.0 (network), 10.0.0.1 and 10.0.0.2 (your two router interfaces), 10.0.0.3 (broadcast). That's it. Two usable addresses, one per end of a point-to-point link. This is the canonical WAN link subnet. Every router-to-router serial link in a Cisco lab uses /30 for exactly this reason.
💡 Good to know: For router-to-router P2P links in modern networks, RFC 6164 introduced /127 as a more efficient option (no subnet-router anycast ambiguity). But /30 remains overwhelmingly standard in production networks and is what you'll see in every CCNA exam question about WAN links.
The Complete CIDR Cheat Sheet — /8 through /32
Memorize /24 first. Then /16 and /30. The rest you look up. That's what this table is for.
Prefix
Subnet Mask
Total IPs
Usable Hosts
Block Size
Common Use
/8
255.0.0.0
16,777,216
16,777,214
16.7M
Large ISP allocation
/16
255.255.0.0
65,536
65,534
65,536
Corporate campus, AWS VPC
/20
255.255.240.0
4,096
4,094
4,096
Large enterprise segment
/22
255.255.252.0
1,024
1,022
1,024
AWS VPC subnets, EKS
/23
255.255.254.0
512
510
512
Medium office floor
/24
255.255.255.0
256
254
256
Home, office LAN, standard
/25
255.255.255.128
128
126
128
Half of /24
/26
255.255.255.192
64
62
64
Department subnet
/27
255.255.255.224
32
30
32
Small office segment
/28
255.255.255.240
16
14
16
Small AWS subnet
/29
255.255.255.248
8
6
8
Very small LAN
/30
255.255.255.252
4
2
4
Point-to-point WAN link
/31
255.255.255.254
2
2
2
P2P link (RFC 3021)
/32
255.255.255.255
1
1
1
Loopback, host route
Where You Actually Use CIDR: Cloud, Home, and Enterprise
Cloud networks (AWS, Azure, GCP) are entirely built on CIDR. Every VPC, subnet, and security group rule uses CIDR notation. AWS VPCs typically start with a /16 (65,536 IPs), which you then carve into /24 subnets for different availability zones and services. AWS reserves 5 IPs per subnet instead of 2 — so a /28 gives you only 11 usable, not 14. Always account for this when sizing cloud subnets.
Home networks almost universally use 192.168.0.0/24 or 192.168.1.0/24. Your router assigns itself .1 and hands out .2 through .254 to your devices via DHCP. The /24 is overkill for most homes — 254 addresses for 30 devices — but the waste doesn't matter since it's private space. If you're setting up a home lab with VLANs, you might use /25 or /26 blocks to separate device types.
Enterprise networks use CIDR extensively for routing efficiency. Instead of advertising 256 separate /24 routes to the internet, a company with 256 subnets can summarize them all into one /16 route. This is called route aggregation or supernetting — it keeps routing tables small and makes BGP manageable. Without CIDR, the global routing table would have collapsed long ago.
Related Calculators — Tools That Work With CIDR
CIDR is the foundation, but real network planning usually needs several of these tools together.
🧮
VLSM Calculator
When you need subnets of different sizes from one larger block — VLSM allocates exactly what each requirement needs, minimizing waste. Essential for CCNA exam questions and real IP addressing plans.
🧱
IPv6 Subnet Calculator
CIDR notation in IPv6 works identically in concept, but addresses are 128 bits. /64 is the standard LAN subnet size. This calculator handles the expansion, compression, and network calculations for any IPv6 prefix.
☁️
AWS VPC Subnet Calculator
AWS reserves 5 IPs per subnet, not 2. This calculator adjusts for that and helps you plan multi-AZ VPC architectures with the right subnet sizes for your workload across availability zones.
🔒
Wildcard Mask Calculator
The inverse of a subnet mask — used in Cisco ACLs, OSPF network statements, and EIGRP. When you need to match a range of IPs in an access list, this converts your CIDR notation into the wildcard format those commands expect.
These are the errors that trip up engineers at every level — from CCNA students to people who've been doing this for years.
⚠️ Mistake 1: Confusing total IPs with usable hosts. A /24 has 256 total IPs but only 254 usable. The network address and broadcast address are reserved and cannot be assigned to devices. In AWS, it's even tighter — 5 reserved, so a /24 gives you 251 usable. Always subtract the reserved addresses before sizing a subnet.
⚠️ Mistake 2: Assigning the network address or broadcast address to a host. Configuring 192.168.1.0 as a device's IP on a /24 network will cause strange behavior — the device may appear to respond but will have routing issues. The network address (first IP) and broadcast address (last IP) are never assigned to devices. This is the most common lab mistake students make.
⚠️ Mistake 3: Forgetting that CIDR prefix and subnet mask are the same thing in different formats. /24 and 255.255.255.0 are identical. If a firewall asks for a subnet mask and you type /24, it will reject it. If a routing protocol config needs CIDR notation and you enter 255.255.255.0, it won't work. Know which format each system expects.
⚠️ Mistake 4: Using /30 for IPv6 point-to-point links. In IPv4, /30 is the standard P2P subnet. In IPv6, you should use /127 (RFC 6164) instead. Using /126 in IPv6 creates subnet-router anycast address complications. The IPv4 habit of /30 does not translate directly to IPv6.
Frequently Asked Questions
CIDR (Classless Inter-Domain Routing) is a compact notation that combines an IP address with a prefix length into a single string. Format: IP/prefix, like 192.168.1.0/24. The prefix number tells you how many bits identify the network. CIDR replaced the old Class A/B/C system in 1993 and is now the universal standard for IP addressing and routing.
256 total, 254 usable. A /24 means 8 host bits, so 2^8 = 256 total IPs. Subtract the network address (first IP) and broadcast address (last IP) and you get 254 usable host addresses. Example: 192.168.1.0/24 gives you 192.168.1.1 through 192.168.1.254 as assignable addresses.
They express the same information differently. /24 equals 255.255.255.0. /16 equals 255.255.0.0. /8 equals 255.0.0.0. CIDR is more compact and used in routing tables, cloud configs, and documentation. Subnet mask (dotted decimal) is used in older device interfaces and Windows ipconfig output. They are interchangeable — convert between them by counting the 1-bits in the binary of the subnet mask.
For 192.168.1.50/24: (1) Convert /24 to mask = 255.255.255.0. (2) Network = 192.168.1.50 AND 255.255.255.0 = 192.168.1.0. (3) Wildcard = 0.0.0.255. (4) Broadcast = 192.168.1.0 OR 0.0.0.255 = 192.168.1.255. (5) Usable: .1 to .254 (254 hosts). Use the calculator above to get all values instantly for any IP and prefix.
65,536 total, 65,534 usable. 2^16 = 65,536. Subnet mask: 255.255.0.0. Example: 10.0.0.0/16 covers 10.0.0.0 through 10.0.255.255. This is a common size for corporate networks, AWS default VPCs (172.31.0.0/16), and any environment that needs to be carved into many smaller /24 subnets.
Point-to-point router links. A /30 has 4 IPs and only 2 usable host addresses — one per router interface. Example: 10.0.0.0/30 gives 10.0.0.1 and 10.0.0.2 as usable addresses. The minimal footprint is the point: when you only need two devices to communicate, don't waste a /24.
The /24 means the first 24 bits of the address identify the network and the remaining 8 bits identify individual hosts. In binary: 11111111.11111111.11111111.00000000 = 255.255.255.0. The 8 remaining host bits allow 256 addresses (254 usable). /24 is the most common subnet size on earth — every home router default network is a /24.
Bitwise AND the IP address with the subnet mask. For 192.168.1.75/24: 192.168.1.75 AND 255.255.255.0 = 192.168.1.0. The network address is the first address in the subnet. It cannot be assigned to any device. Every device on the same /24 shares the same network address: x.x.x.0.
The broadcast address is the last IP in a subnet — all host bits set to 1. For 192.168.1.0/24: broadcast = 192.168.1.255. Packets sent to the broadcast address go to every device on the subnet. Like the network address, the broadcast address cannot be assigned to a device. Calculated by ORing the network address with the wildcard mask.
Start with /16 for the VPC itself (65,536 IPs) and use /24 subnets within it. AWS reserves 5 IPs per subnet (not 2 like standard subnetting), so a /24 gives 251 usable IPs and a /28 gives only 11. Use /20 or /22 for production subnets that need room to grow. You cannot resize subnets after creation, so plan conservatively.
Every subnet reserves two addresses: the network address (first IP, all host bits zero) and the broadcast address (last IP, all host bits one). These cannot be assigned to devices. So a /24 with 256 total has 254 usable, a /26 with 64 total has 62 usable. Exception: /31 per RFC 3021 has 2 total and 2 usable (no broadcast, used for P2P links). /32 is a single host address.
Before CIDR, every IPv4 network was Class A (/8, 16.7M hosts), Class B (/16, 65K hosts), or Class C (/24, 254 hosts). A company needing 500 hosts got a Class B with 65,534 IPs — almost 65,000 addresses wasted. CIDR allows any prefix from /0 to /32, so you can allocate exactly the right size. It's why IPv4 is still running in 2026 despite predictions of exhaustion by 2000.
A wildcard mask is the bitwise inverse of a subnet mask: 255.255.255.255 minus subnet mask. For /24 (mask 255.255.255.0): wildcard = 0.0.0.255. Used in Cisco ACL syntax and OSPF network statements. A 0-bit means "must match this bit exactly" and a 1-bit means "ignore this bit." It is not used in standard host configuration — only in ACL and routing protocol contexts.