AWS CIDR & Subnet Mask | Overview.

AWS CIDR & Subnet Mask - Overview.
Scope:

• Intro,
• Core AWS CIDR Constraints,
• Reserved IP Addresses,
• Table for Common CIDR to Subnet Mask Conversions,
• Links to Detailed documentation,
• The Concept of CIDR & Subnet Masks (Definitions),
• Table for CIDR vs Subnet Mask Relationship,
• How CIDR Defines Subnet Boundaries,
• Table for CIDR & Subnet Mask in AWS VPCs (Sample VPC design),
• Key Insights.

Intro:

• In AWS, Classless Inter-Domain Routing (CIDR) notation is the standard method for defining IP address ranges for Virtual Private Clouds (VPCs)  & subnets. 
• Classless Inter-Domain Routing (CIDR) notation replaces traditional IP classes with a flexible system where a slash is followed by a number (the subnet mask or prefix length).
• The subnet mask or prefix length indicates how many bits represent the network portion of the address. 

Core AWS CIDR Constraints 

• AWS enforces specific size limits for IPv4 CIDR blocks: 
• Allowed block sizes range from a /16 (65,536 addresses) to a /28 (16 addresses).
• A VPC CIDR cannot be changed once created, though additional secondary CIDR blocks can be added.
• Subnet CIDR blocks must be a subset of the VPC's CIDR block and cannot overlap with other subnets in the same VPC.

Reserved IP Addresses

• In every AWS subnet, five IP addresses are reserved and cannot be assigned to resources like EC2 instances: 

1. Network address (e.g., 10.0.0.0)
2. VPC router (e.g., 10.0.0.1)
3. DNS server (e.g., 10.0.0.2)
4. Future use (e.g., 10.0.0.3)
5. Network broadcast address (e.g., 10.0.0.255)

Table for Common CIDR to Subnet Mask Conversions

CIDR	Subnet Mask	Total Addresses	Usable AWS Addresses
/16	255.255.0.0	65,536	65,531
/20	255.255.240.0	4,096	4,091
/24	255.255.255.0	256	251
/28	255.255.255.240	16	11

Links to Detailed documentation:

• Subnet sizing

https://docs.aws.amazon.com/vpc/latest/userguide/subnet-sizing.html

• VPC CIDR blocks 

https://docs.aws.amazon.com/vpc/latest/userguide/vpc-cidr-blocks.html

NB:

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

The Concept of CIDR & Subnet Masks (Definitions).

•      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).

 Table for 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

Sample: 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

·       To 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

Table for CIDR & Subnet Mask in AWS VPCs (Sample 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