Engine Yard differentiates between a public hostname and an elastic IP (EIP) address (often referred to as an IP address). The public hostname is automatically assigned to your server and includes a DNS domain and the IP address of the host it represents. Public hostnames are created by Amazon and automatically assigned when you provision a server. Engine Yard considers it best practices to use an EIP for mapping because of its durability, rather than a public hostname which changes if the VM is rebooted. An EIP address is optional, typically needed for whitelisting purposes, and belongs to the account and not to a virtual machine instance.
Elastic IP addresses are similar to static IP addresses in traditional data centers, with one key difference. EIP addresses allow you to mask instance or availability zone failures by programmatically remapping your public IP addresses to any instance associated with your account. You can map an elastic IP address to any virtual machine instance without the help of a network administrator and without having to wait for DNS to propagate the new binding. In this sense, EIPs belong to the account and not to a virtual machine instance. An EIP address can only be used within its provisioned region.
Environments and IP Addresses
An EIP is assigned when you configure an environment. The following information applies to the Engine Yard environment and IP addresses:
An EIP is the default selection when you are setting up your environment.
For a single instance environment, an EIP address is not typically required. If you choose to have an EIP address for your single instance environment, the EIP address is assigned to this one instance. All incoming traffic is routed to this instance.
When deploying a multi-instance environment, the EIP is assigned to the application master instance. All incoming internet traffic is routed to this instance which is then load balanced to your application slave instances with HAProxy.
Staging and development environments do not necessarily require EIP addresses.
Each Engine Yard account has a default of 5 EIP addresses per region. Contact Engine Yard Support to request for more addresses.
This table provides other resources related to app slave IP addresses.