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

Intro:

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

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

 Breakdown:

•        The concept:CIDR (Classless Inter-Domain Routing),
•        Structure of an IPv4 CIDR Block,
•        CIDR Notation Breakdown,
•        How to Calculate a CIDR Block,
•        CIDR Aggregation (Supernetting)
•        CIDR in AWS VPC Design,
•        Tools & Tips for CIDR Planning,
•        CIDR in Security Context (AWS SGs & NACLs),
•        Advanced Topics.

The concept: 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 and flexible.

 Structure of an IPv4 CIDR Block

A CIDR block is written in the format:

<IPv4 Address>/<Prefix Length>

Example:

192.168.10.0/24

From above:

•         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

 How to Calculate a CIDR Block

Example:
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

(Usable = 65,534, because 1 for network + 1 for broadcast)

CIDR Aggregation (Supernetting)

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

Example: The 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.

Example:

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 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

Examples:

0.0.0.0/0       → open to the internet

10.0.0.0/16     → internal network

203.0.113.0/24  → specific corporate range

 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.

How to calculate CIDR to IP Range from the website: https://www.ipaddressguide.com/cidr

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.