AWS Classless Inter-Domain Routing (CIDR) for IPv4 | Overview.

AWS Classless Inter-Domain Routing (CIDR) for IPv4 - Overview.

Scope:

•       Intro,
•       VPC CIDR Block Requirements.
•       Subnet CIDR Sizing & Reservations, 
•       Table of Common CIDR Samples (Notation, Total addresses & Assignable Addresses),  
•       The concept of CIDR (Classless Inter-Domain Routing),
•       Structure of an IPv4 CIDR Block (format),
•       Sample CIDR Block,
•       CIDR Notation Breakdown,
•       Explanation CIDR Block,
•       Sample Calculation for a CIDR Block (when twtech is given 10.0.0.0/16) ,
•      CIDR Aggregation (Supernetting),
•       Sample CIDR Aggregation (Supernetting) for network,
•       CIDR in AWS VPC Design,
•       Tools & Tips for CIDR Planning,
•       CIDR in Security Context (AWS SGs & NACLs),
•       Samples CIDR in Security Context (AWS SGs & NACLs)
•       Architecture,
•       Link to calculate CIDR to IP Range from the website,
•       Insights.

Intro:

• In AWS, Classless Inter-Domain Routing (CIDR) is the notation used to define the range of IPv4 addresses for a Virtual Private Cloud (VPC) and its subnets. 
• Classless Inter-Domain Routing (CIDR) represents an IP address followed by a slash and a number (the prefix length).
• The prefix length indicates the number of bits used for the network portion of the address. 

VPC CIDR Block Requirements

• When creating a VPC, twtech must specify a primary IPv4 CIDR block.
• Size Limits: The allowed block size is between a /16 netmask (65,536 addresses) and a /28 netmask (16 addresses).
• Private Ranges: AWS recommends using private IPv4 address ranges from RFC 1918, such as 10.0.0.0/16, 172.31.0.0/16, or 192.168.0.0/20.
• Secondary CIDRs: twtech can associate additional (secondary) CIDR blocks after creation to expand the VPC.
•  But twtech cannot change the size of an existing block. 

Subnet CIDR Sizing & Reservations

• Subnet CIDR blocks must be a subset of the VPC's CIDR blocks and cannot overlap with other subnets in the same VPC.
• Subnet Size: Like VPCs, the size must be between /16 and /28.
• Reserved Addresses: In every subnet, AWS reserves exactly five IP addresses for internal networking.

NB:

• The reserved five IP addresses  cannot be assigned to resources like EC2 instances. 
• The reserved five IP addresses are used for:

1. Base address (e.g., .0): Network address.
2. Base + 1 (e.g., .1): Reserved for the VPC router.
3. Base + 2 (e.g., .2): Reserved for the AWS DNS server.
4. Base + 3 (e.g., .3): Reserved by AWS for future use.
5. Last address (e.g., .255): Network broadcast address.

 NB:

• AWS does not support broadcast address but still reserves this. 

Table of Common CIDR Samples (Notation, Total addresses & Assignable Addresses)

Notation	Total Addresses	Assignable Addresses (Total - 5)
/16	65,536	65,531
/20	4,096	4,091
/24	256	251
/28	16	11

• CIDR (Classless Inter-Domain Routing) for IPv4 is a fundamental concept in networking, especially when designing:
• VPCs, 
• Subnets,
• Routing in AWS or any cloud environment.

The concept of CIDR (Classless Inter-Domain Routing)

•     CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing Internet Protocol packets.
•     CIDR (Classless Inter-Domain Routing)  replaced the old classful addressing system (Class A, B, C) to make IP address allocation more efficient & flexible.

 Structure of an IPv4 CIDR Block (format):

<IPv4 Address>/<Prefix Length>

Sample CIDR Block 

192.168.10.0/24

Explanation CIDR Block:

• 192.168.10.0 → Network address
• /24 → Prefix length (number of bits representing the network)
• The remaining (32 - 24 = 8) bits are reserved for host addresses.

 CIDR Notation Breakdown

CIDR	Network Bits	Host Bits	# of Hosts (Usable)	Subnet Mask	Sample Range
/8	8	24	16,777,214	255.0.0.0	10.0.0.0 – 10.255.255.255
/16	16	16	65,534	255.255.0.0	192.168.0.0 – 192.168.255.255
/24	24	8	254	255.255.255.0	192.168.1.0 – 192.168.1.255
/28	28	4	14	255.255.255.240	192.168.1.0 – 192.168.1.15

 Sample Calculation for a CIDR Block (when twtech is given 10.0.0.0/16) 

• Network bits: 16
• Host bits: 16 → (32 – 16 = 16)
• Number of addresses: 2^16 = 65,536 total addresses

NB:

• Usable IP addresses = 65,534, 
• Because:
•  One IP address is reserved for network.
•  Another IP address is reserved for broadcast,

CIDR Aggregation (Supernetting)

• CIDR allows grouping multiple contiguous networks into a single routing entry.

Sample CIDR Aggregation (Supernetting) for networks.

192.168.0.0/24

192.168.1.0/24

This Can be summarized as:

192.168.0.0/23

NB:

• This reduces routing table entries — critical in internet routing.

 CIDR in AWS VPC Design

• In AWS, twtech defines the VPC CIDR block, then subdivide it into subnets (Sample):

VPC CIDR: 10.0.0.0/16

Subnets:

• Public Subnet A → 10.0.0.0/24
• Private Subnet A → 10.0.1.0/24
• Public Subnet B → 10.0.2.0/24
• Private Subnet B → 10.0.3.0/24

  NB:

This allows for isolation and controlled routing.

 Tools & Tips for CIDR Planning

• CIDR Calculator:
• Tols like cidr.xyz, ipcalc, or AWS VPC wizard help in visualizing ranges.
• Avoid Overlap:
• Ensure CIDR ranges don’t overlap when peering or connecting networks.
• Private IPv4 Ranges (RFC 1918):
•    10.0.0.0/8 → Large enterprises
•    172.16.0.0/12 → Mid-size
•    192.168.0.0/16 → Small networks

 CIDR in Security Context (AWS SGs & NACLs)

• CIDR is used to define source/destination IP ranges in:
• Security Groups
• Network ACLs
• Route Tables

Samples CIDR in Security Context (AWS SGs & NACLs):

0.0.0.0/0       → open to the internet

10.0.0.0/16     → internal network

203.0.113.0/24  → specific corporate range

Architecture: 

 Link to calculate CIDR to IP Range from the website: 

https://www.ipaddressguide.com/cidr

Insights: Advanced Topics

• VLSM (Variable Length Subnet Masking):
• Allows efficient IP allocation using variable subnet sizes.
• Route Summarization:
• Reduces route table size by combining multiple CIDRs.
• CIDR Overlaps in Multi-VPC / Hybrid Cloud:
• Must plan carefully to avoid conflicts with on-premises IP space.
• IPv6 Equivalent:
• Uses /n notation as well, but supports 128-bit addressing.

CIDR to IPv4 Conversion

•  CIDR is the short for Classless Inter-Domain Routing, an IP addressing scheme that replaces the older system based on classes A, B, and C.
•  A single IP address can be used to designate many unique IP addresses with CIDR.
•  A CIDR IP address looks like a normal IP address except that it ends with a slash followed by a number, called the IP network prefix.
• CIDR addresses reduce the size of routing tables and make more IP addresses available within organizations.