Elastic IP

A deep dive into Elastic IP - Practical ver.

Elastic IP == Static IP

• When an EC2 Instance's Elastic IP field is empty but Public IPv4 has a value

  • The IP changes when the instance is stopped & started

    • Assigning a value to Elastic IP allows you to use a fixed IP without this issue!

      • However, you cannot create an Elastic IP based on a previously assigned Public IP

        • You should set up the Elastic IP when initially creating the instance!

Elastic IP Charges

• AWS charges you if an Elastic IP is not assigned to anything!

  • If it's attached to an EC2, there's no charge

    • Why?

      • AWS's reasoning is: "IPv4 is a finite resource, so we charge to encourage conservation when it's not in use"

        • However, if it's attached to an EC2, you're already paying for the instance, so they don't charge additionally

Elastic IP address limit

• The number of Elastic IPs you can own per account is limited

  • Since IPv4 is a finite resource, AWS won't provide too many if you request excessively

  • By default, you can own 5 Elastic IPs per region

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Public IP

• Just because something is in a public subnet doesn't mean it must have a public IP!

  • But you probably created it in a public subnet because you need public connectivity, right?

• If an EC2 exists in a public subnet without a Public IP

  • External to internal access is not possible

  • Internal to external access is also not possible

• The reason EC2 instances in a private subnet can access the outside even without a Public IP is because they go through the NAT's Public IP

• The reason SSH access works even without a Public IP

  • It's because firewall settings are configured