What is CIDR notation and how does it work?

CIDR (Classless Inter-Domain Routing) notation represents an IP address and its associated subnet mask prefix length using a slash followed by the number of network bits (e.g., 192.168.1.0/24). The /24 indicates 24 network bits (255.255.255.0) and 8 host bits, yielding 256 total IP addresses and 254 usable hosts.

Why are two IP addresses subtracted in every IPv4 subnet?

In standard IPv4 subnets, the very first address with all host bits set to 0 is reserved as the Network Identifier, and the last address with all host bits set to 1 is reserved as the Directed Broadcast Address. Hence, usable hosts = (2^(32 - prefix)) - 2.

How do AWS VPC subnets differ in usable IP calculations?

In AWS VPCs, Amazon reserves 5 IP addresses per subnet rather than 2: the Network address (.0), VPC router (.1), DNS server (.2), future AWS reservation (.3), and Broadcast address (.255). For a /28 in AWS, usable IPs = 16 - 5 = 11.

🎮 100% Free DevOps & Cloud Speed Drill

CIDR & Subnetting Speed Challenge

Q: How do you calculate network and broadcast addresses in under 3 seconds?

Use the Magic Number formula: Magic Number = 256 - mask of interesting octet. For /27, mask is 224, giving 256 - 224 = 32. Subnets increment by 32. For IP 10.0.0.75/27, the nearest lower multiple of 32 is 64, making 10.0.0.64 the network address, and 64 + 32 - 1 = 95 the broadcast address.

Q: What are RFC 1918 Private IP address ranges?

RFC 1918 designates three non-routable private IP ranges: 10.0.0.0/8 (10.0.0.0 - 10.255.255.255 with 16.7M IPs), 172.16.0.0/12 (172.16.0.0 - 172.31.255.255 with 1M IPs), and 192.168.0.0/16 (192.168.0.0 - 192.168.255.255 with 65,536 IPs). These are reserved for internal corporate networks and cloud VPCs.

Master IPv4 subnet masks, usable host calculations, and VPC CIDR design under time pressure. Built for DevOps engineers, SREs, and CCNA/AWS candidates.

Time Remaining 60s

Current Score 0

Streak 0 🔥

Questions 1 / 10

What is the Subnet Mask for:

192.168.1.0/26

Select the matching 32-bit dotted-decimal mask

💡 Explanation:

/26 leaves 6 host bits (2^6 = 64 total addresses). Subnet mask: 256 - 64 = 255.255.255.192.

🏆

Session Complete!

3,450 PTS

Accuracy

90%

Max Streak

7 🔥

Speed/Ans

2.4s

☸️ K8s Diagnostic Quiz

Click individual bits below to toggle between 1 (network) and 0 (host). Watch the CIDR, Subnet Mask, and Usable Hosts calculate in real time.

Dotted Subnet Mask

255.255.255.0

CIDR Prefix

/24

Wildcard Mask

0.0.0.255

Total IP Addresses

256

Usable Host IPs

254

☁️ Scenario: AWS 3-Tier Multi-AZ VPC Design

You have been assigned the CIDR block 10.0.0.0/22 (1,024 total IPs). You need to allocate non-overlapping subnets to satisfy the infrastructure capacity requirements below.

1. Public Web Subnet

Needs at least 100 usable IPs

2. Private App Subnet

Needs at least 200 usable IPs

3. Isolated DB Subnet

Needs at least 25 usable IPs

⚡ IPv4 Subnetting & CIDR Cheat Sheet

Quick reference table for common subnet masks used in production Kubernetes CNI planning, AWS VPCs, and Docker overlay networks.

CIDR Prefix	Subnet Mask	Wildcard Mask	Total IP Addresses	Usable Hosts (RFC 1918)	AWS VPC Usable
`/32`	255.255.255.255	0.0.0.0	1	1 (Host route)	0
`/30`	255.255.255.252	0.0.0.3	4	2 (Point-to-Point)	0 (AWS min is /28)
`/29`	255.255.255.248	0.0.0.7	8	6	0
`/28`	255.255.255.240	0.0.0.15	16	14	11 (16 - 5)
`/27`	255.255.255.224	0.0.0.31	32	30	27 (32 - 5)
`/26`	255.255.255.192	0.0.0.63	64	62	59 (64 - 5)
`/25`	255.255.255.128	0.0.0.127	128	126	123 (128 - 5)
`/24`	255.255.255.0	0.0.0.255	256	254	251 (256 - 5)
`/23`	255.255.254.0	0.0.1.255	512	510	507
`/22`	255.255.252.0	0.0.3.255	1,024	1,022	1,019
`/20`	255.255.240.0	0.0.15.255	4,096	4,094	4,091
`/16`	255.255.0.0	0.0.255.255	65,536	65,534	65,531

How the Math Works in Your Head

The secret to fast mental subnetting is remembering the Magic Number: Magic Number = 256 - Interesting Octet. For instance, in /27, the last octet mask is 224. 256 - 224 = 32. The subnets jump in increments of 32: .0, .32, .64, .96, .128... Any IP between .0 and .31 belongs to the .0 network with broadcast .31!

❓ Frequently Asked Questions

What is CIDR notation and why do DevOps engineers need it? +

CIDR (Classless Inter-Domain Routing) replaced legacy Class A, B, and C networking. DevOps engineers use CIDR everyday to configure Kubernetes Pod and Service CIDRs (Calico, Flannel, AWS VPC CNI), provision Terraform VPC modules, write security group rules, and prevent IP exhaustion across microservices.

Why does AWS reserve 5 IP addresses per subnet instead of 2? +

Standard RFC networking reserves 2 addresses: Network (.0) and Broadcast (.255). AWS reserves 3 additional addresses inside every subnet: .1 for the VPC Default Router, .2 for the Amazon DNS server, and .3 for future AWS internal use. Keep this in mind when sizing subnets for EKS or ECS clusters!

How does the 60-Second Speed Blitz work? +

You have 60 seconds to answer as many randomized subnetting questions as possible. Correct answers award points and build your streak multiplier. Wrong answers deduct 3 seconds from your timer and reset your combo.

How do you calculate network and broadcast addresses in under 3 seconds? +

Use the Magic Number formula: Magic Number = 256 - mask of interesting octet. For a /27 subnet, the mask is 224, giving 256 - 224 = 32. Subnets increment by 32. For IP 10.0.0.75/27, the nearest lower multiple of 32 is 64, making 10.0.0.64 the network address, and 64 + 32 - 1 = 10.0.0.95 the broadcast address!

What are RFC 1918 Private IP address ranges? +

RFC 1918 designates three non-routable private IP ranges:

10.0.0.0/8 (10.0.0.0 – 10.255.255.255, 16.7M IPs) — Used for large enterprise networks and complex cloud VPC topologies.
172.16.0.0/12 (172.16.0.0 – 172.31.255.255, 1M IPs) — Default range for Docker bridge networks and Kubernetes pod CIDRs.
192.168.0.0/16 (192.168.0.0 – 192.168.255.255, 65,536 IPs) — Common home, lab, and small office networks.

How to size subnets to prevent IP exhaustion in Kubernetes EKS and GKE clusters? +

In Kubernetes clusters using AWS VPC CNI, each pod receives an actual routable IP from the worker node subnet. Sizing subnets at /24 (251 usable IPs in AWS) per availability zone ensures adequate headroom for 50–100 pods and node auto-scaling. Subnets smaller than /26 frequently cause FailedCreatePodSandBox outages during traffic spikes.

Is this Subnetting Game free to use? +

Yes! The CIDR & Subnetting Challenge is 100% free, runs entirely inside your browser with zero logins or tracking, and works seamlessly on mobile, tablet, and desktop.