Elastic IP Addresses and ENIs

EC2 networking goes beyond basic IP assignment. Elastic IP addresses provide stable public endpoints, and Elastic Network Interfaces (ENIs) give you fine-grained control over how instances connect to the network. This lesson covers both, along with practical scenarios and best practices.

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Elastic IP Addresses (EIPs)

What Is an Elastic IP?

An Elastic IP is a static, public IPv4 address that you allocate to your AWS account and can associate with any EC2 instance or network interface in the same Region.

Unlike auto-assigned public IPs (which change every time you stop and start an instance), an Elastic IP remains the same until you explicitly release it.

Why Use Elastic IPs?

Scenario	Without EIP	With EIP
Instance restart	Public IP changes; DNS records become stale	Public IP stays the same
Instance failure	Must update all references to the new IP	Remap EIP to a replacement instance in seconds
Multi-homed instances	Limited to one public IP	Assign multiple EIPs to different ENIs

Allocating and Associating an EIP

# Allocate an Elastic IP
aws ec2 allocate-address --domain vpc
# Returns: AllocationId = eipalloc-0abcdef1234567890, PublicIp = 203.0.113.25

# Associate with an instance
aws ec2 associate-address \
  --allocation-id eipalloc-0abcdef1234567890 \
  --instance-id i-0123456789abcdef0

# Disassociate from an instance
aws ec2 disassociate-address \
  --association-id eipassoc-0abcdef1234567890

# Release the EIP back to the pool
aws ec2 release-address \
  --allocation-id eipalloc-0abcdef1234567890

EIP Pricing

Condition	Cost
EIP associated with a running instance	Free (first EIP per instance)
Additional EIPs on same instance	Charged per hour
EIP not associated with any instance	Charged per hour (idle EIP)
EIP on a stopped instance	Charged per hour
Data transfer through EIP	Standard data transfer rates

Important: AWS charges for idle Elastic IPs to discourage hoarding of scarce IPv4 addresses. Always release EIPs you are not using.

EIP Limits and Quotas

By default, each AWS account is limited to 5 Elastic IPs per Region. You can request an increase through the Service Quotas console, but AWS encourages alternatives:

• Use a load balancer (ALB/NLB) instead of assigning EIPs to individual instances
• Use DNS names (Route 53 or other DNS providers) with auto-assigned public IPs
• Use AWS Global Accelerator for static anycast IP addresses

EIP Best Practices

1. Prefer load balancers over EIPs for production workloads — they provide high availability and SSL termination.
2. Release unused EIPs — avoid charges and IPv4 waste.
3. Use tags to track which team, project, or environment owns each EIP.
4. Automate failover — use scripts or Lambda functions to remap EIPs during instance replacement.
5. Consider IPv6 — IPv6 addresses are static by default and do not require Elastic IPs.

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Elastic Network Interfaces (ENIs)

What Is an ENI?

An Elastic Network Interface is a virtual network card that you attach to an EC2 instance. Every instance has at least one ENI (the primary ENI, eth0), and you can attach additional ENIs for advanced networking scenarios.

ENI Attributes

Each ENI carries several network attributes: