Intro to IP Addressing, Subnetting and CIDR Notation for AWS VPCs
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Amazon Virtual Private Cloud (VPC) allows you to create isolated networks for your resources in AWS. To effectively design and manage these networks, you need to understand IP addresses, CIDR notation, and subnetting. These concepts are important to understand for allocating resources, controlling traffic, and ensuring scalability.
While you can use online IP subnet calculators, you must also understand what's happening under the hood.
What is an IP Address?
An IP address (Internet Protocol address) is a unique identifier for devices on a network. It allows devices to locate and communicate with each other over the internet or private networks.
Structure of an IP Address
An IPv4 address (commonly used in AWS) is a 32-bit number divided into four octets, separated by dots, in a format called dotted decimal notation. Each octet represents 8 bits, making the total 32 bits (4 x 8).
For example:192.168.1.1 is an IPv4 address where:
Example Breakdown of 192.168.1.1:
This binary format is essential for subnetting and routing.
What is a Subnet Mask?
A subnet mask defines which portion of an IP address identifies the network and which part identifies the host (the specific device in the network). It helps divide a larger network into smaller subnets.
For example, the subnet mask 255.255.255.0:
This subnet mask corresponds to CIDR notation /24, indicating that the first 24 bits are used for the network.
How IP Addresses, Subnet Masks, and CIDR Work Together
Using the IP address 192.168.1.0/24:
CIDR Notation: Structuring Your IP Space
CIDR (Classless Inter-Domain Routing) notation simplifies the representation of IP address ranges. It combines the IP address and the number of network bits.
Examples of CIDR Notation
| CIDR Block | Subnet Mask | Usable IPs | Typical Use Case |
|---|---|---|---|
10.0.0.0/16 | 255.255.0.0 | 65,536 | Large VPC with multiple subnets. |
192.168.1.0/24 | 255.255.255.0 | 251 | Single subnet for small workloads. |
172.16.0.0/12 | 255.240.0.0 | 1,048,576 | Enterprise-scale private IP range. |
Subnetting: Dividing Your VPC
Subnetting divides your VPC into smaller networks, called subnets, to isolate resources and control traffic flow. Each subnet is associated with a CIDR block, which determines its size.
Why Subnetting Matters
Subnetting in AWS
In AWS, subnets are defined within Availability Zones (AZs). For example, a VPC with the CIDR 10.0.0.0/16 can be divided into smaller subnets:
Each subnet provides usable IP addresses for resources, with AWS reserving the first four and last IP addresses in the range.
Example Breakdown of Subnet IP Allocation:
For the subnet 10.0.1.0/24:
Best Practices for Subnetting and IP Addressing
Summary
An IP address identifies devices within a network, and subnet masks define how to separate networks and hosts. CIDR notation simplifies the representation of IP ranges, while subnetting allows you to structure your VPC for efficiency, security, and scalability.
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