CIDR & Subnet Mask | Overview.

An Overview of CIDR and Subnet Masks.

Intro: Understanding

•        How CIDR and Subnet Masks are related.
•         How subnet masks define network boundaries.
•         How this network boundaries impacts IP design in VPCs and routing.

Breakdown:

•        The Concept: CIDR and Subnet Masks,
•        CIDR vs Subnet Mask Relationship,
•        Subnet Mask Binary Breakdown
•        How CIDR Defines Subnet Boundaries,
•        CIDR & Subnet Mask in AWS VPCs,
•        Key Insights,
•        Quick Conversion Reference.

The Concept: CIDR and Subnet Masks.

·       Both the CIDR and Subnet Masks define how an IP network is divided between network and host portions.

Concept	Definition
CIDR (Classless Inter-Domain Routing)	Expresses the number of bits used for the network prefix (e.g., /24).
Subnet Mask	Binary or dotted-decimal representation of those network bits (e.g., 255.255.255.0).

NB:

•        They’re two ways of expressing the same concept.

 CIDR vs Subnet Mask Relationship

CIDR	Subnet Mask	# of Networks	# of Hosts (Usable)	Example Range
/8	255.0.0.0	1	16,777,214	10.0.0.0 – 10.255.255.255
/16	255.255.0.0	256	65,534	10.0.0.0 – 10.0.255.255
/24	255.255.255.0	65,536	254	10.0.0.0 – 10.0.0.255
/28	255.255.255.240	1,048,576	14	10.0.0.0 – 10.0.0.15

 Subnet Mask Binary Breakdown

•        Each subnet mask octet can be visualized as a series of 1s for network bits and 0s for host bits:

CIDR	Subnet Mask	Binary Representation
/8	255.0.0.0	11111111.00000000.00000000.00000000
/16	255.255.0.0	11111111.11111111.00000000.00000000
/24	255.255.255.0	11111111.11111111.11111111.00000000
/28	255.255.255.240	11111111.11111111.11111111.11110000

NB:

·       The more 1s, the more network bits — smaller subnets, fewer hosts.

 How CIDR Defines Subnet Boundaries

Example: 10.0.0.0/24

•         Network bits: first 24 bits → 10.0.0
•         Host bits: last 8 bits → range for hosts
  
• 10.0.0.1 – 10.0.0.254
•         Total usable hosts: 254

·       If we subnet further to /28, we divide /24 into 16 smaller subnets:

•         10.0.0.0/28 → 10.0.0.0 – 10.0.0.15
•         10.0.0.16/28 → 10.0.0.16 – 10.0.0.31
•         ...
•         10.0.0.240/28 → 10.0.0.240 – 10.0.0.255

 CIDR & Subnet Mask in AWS VPCs

Example VPC design:

Layer	CIDR	Subnet Mask	Purpose
VPC	10.0.0.0/16	255.255.0.0	Entire virtual network
Public Subnet	10.0.1.0/24	255.255.255.0	Internet-facing instances
Private Subnet	10.0.2.0/24	255.255.255.0	Internal workloads
Database Subnet	10.0.3.0/28	255.255.255.240	RDS isolated zone

 Key Insights

• CIDR simplifies representation – /24 is easier than 255.255.255.0.
• Subnet masks are crucial for routers and firewalls, which interpret them at binary level.
• CIDR aggregation (supernetting) improves routing efficiency.
• Overlapping CIDRs cause connectivity conflicts — always plan network ranges carefully.

 Quick Conversion Reference

CIDR	Subnet Mask	Hosts	Networks (per /16)
/17	255.255.128.0	32,766	2
/18	255.255.192.0	16,382	4
/19	255.255.224.0	8,190	8
/20	255.255.240.0	4,094	16
/21	255.255.248.0	2,046	32
/22	255.255.252.0	1,022	64
/23	255.255.254.0	510	128
/24	255.255.255.0	254	256