EC2 Advanced Networking & Features

Private vs. Public vs. Elastic IP

Understanding IP addressing behaviour is critical for the exam, especially regarding instance lifecycle events (Stop/Start).

Private

• Definition: The internal IP address used for communication within the VPC.
• Persistence: Retained for the life of the instance. If you Stop and Start the instance, the Private IP does not change.
• Scope: Only reachable within the VPC or via VPN/Direct Connect (unless NAT is used).

Public

• Definition: An IP address reachable from the internet.
• Behaviour:
  • Assigned automatically if Auto-assign Public IP is enabled in the subnet settings or requested at launch.
  • Dynamic: If you Stop and Start the instance, the Public IP will change.
  • Hostname: Cannot be manually moved between instances.

Elastic IP (EIP)

• Definition: A static, fixed Public IP address allocated to your AWS account.
• Use Case: Masking instance failures by rapidly remapping the address to another instance.
• Behaviour:
  • You own it until you release it.
  • Static: It does not change if you stop/start the instance.
  • Scope: Region-specific (cannot move an EIP from us-east-1 to us-west-2).
• Cost:
  • Free if attached to a running instance and it is the only EIP attached.
  • Charges apply if: The instance is stopped, the EIP is unattached, or you have more than one EIP attached to a running instance.

EC2 Placement Groups

Placement groups determine how instances are placed on underlying hardware to optimize for performance, durability, or availability.

Elastic Network Interfaces (ENI) - Overview

An ENI is a logical networking component in a VPC that represents a virtual network card.

• Scope: Bound to a specific Availability Zone (AZ). You cannot attach an ENI created in AZ A to an instance in AZ B.
• Attributes: An ENI can have:
  • A primary private IPv4 address.
  • One or more secondary private IPv4 addresses.
  • One Elastic IP address (per private IPv4).
  • One public IPv4 address.
  • One or more security Groups
  • A MAC address.

Types of ENI Attachment

ENI - Extra Reading

• Failover Scenarios: You can build a low cost failover solution with a secondary ENI that holds a private IP and an Elastic IP. If the primary instance fails, move the ENI to a standby instance and traffic follows it.
• Dual Homing: You might attach two ENIs to one instance:
  • ENI 1: Connected to a management subnet (for SSH/RDP access).
  • ENI 2: Connected to a data subnet (for application traffic).
• Security Groups: Security Groups are attached to the ENI, not the instance itself. Different ENIs on the same instance can therefore have different security group rules.

EC2 Hibernate

EC2 Hibernate lets you pause an instance and resume it later while preserving the state of the RAM.

How it works: the contents of the RAM are written to the Root EBS Volume. On start, the RAM is reloaded from that disk, so the OS is never actually booted from scratch.

Benefits:

• Much faster boot time than a full Stop/Start.
• Applications do not need to “warm up” or reload their caches.

Requirements

Lifecycle & Limits