Elastic Network Interface (ENI)
Elastic Network Interfaces are virtual network interfaces assigned to an EC2 instance within a VPC. ENI supports multiple private IP addresses assignable per network interface. In addition tenants can assign a single public IPv4 address or public Elastic IP to the same interface. In the context of VPC architecture, EC2 instances could include servers or virtual appliances. ENI supports multiple IPv6 addresses, security groups, MAC address and source/destination check attribute.
Elastic IP (EIP)
The public Elastic IP address (EIP) is assigned to the network interface of a cloud web server for example so it is available on the internet. EIP is a static public address that is persistent (remains assigned) even when the instance is stopped. The support for private and public addressing allows for subnetting and security zones. The tenant is assigned a maximum of five static Elastic IP (EIP) addresses per region. There are often multiple public web servers deployed across availability zones. Tenants can enable NAT to conserve addresses or request some additional EIP addresses from Amazon AWS. Note as well that AWS does not currently support EIP for IPv6 addressing.
Elastic IP vs Public IPv4 Address
Public IPv4 addresses are not persistent or reassignable to a different EC2 instance or interface. Amazon AWS returns public IPv4 addresses to a shared pool when an EC2 instance is stopped. Any assigned private IP addressing is persistent however. EIP is released from an EC2 instance when it is terminated.
Elastic IP (EIP) IP is a static internet routable IP address that is persistent when an EC2 instance is stopped. It can be manually reassigned to another instance when there is an instance failure or stopped. There is a nominal charge for EIP while an EC2 instance is stopped. Amazon automatically assigns a public IPv4 address to an instance network interface when the tenant disassociates an EIP from it. In addition the public IPv4 address is released when an EIP is assigned to the network interface.
Elastic IP addresses are statically assigned to an AWS tenant account in contrast to public IPv4 addresses that are allocated from a pool. The primary private IP address assigned to each instance cannot be reassigned. They are assignable with DHCP or can be manually assigned.
An Elastic IP address does not incur charges for the following:
- EC2 instance associated with the EIP is running
- EC2 instance has only one Elastic IP address attached
- EIP address is something you brought to AWS (Bring Your Own IP)
- If you associate additional EIPs with that EC2 instance, you will be charged per hour pro-rated for each additional EIP associated.
There is an hourly charge when an EIP address is are not associated with a running instance, stopped instance or unattached network interface.
$0.005 per additional IP address
$0.005 per EIP address not associated with a running instance
$0.00 per EIP address remap for the first 100 remaps per month
$0.10 per EIP address remap for additional remaps over 100 per month