What do you mean by Elastic IP Addressing? Describe how Elastic IPs works in cloud service.

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 Elastic IP address

  • An Elastic IP address is a static IPv4 address designed for dynamic cloud computing. An Elastic IP address is associated with your AWS account. With an Elastic IP address, you can mask the failure of an instance or software by rapidly remapping the address to another instance in your account.
  •  An Elastic IP address is a public IPv4 address, which is reachable from the Internet. If your instance does not have a public IPv4 address, you can associate an Elastic IP address with your instance to enable communication with the Internet; for example, to connect to your instance from your local computer. 
  • Elastic IP addresses are used by AWS to manage its dynamic cloud computing services. Within the AWS infrastructure, customers have virtual private clouds (VPCs). Within the VPCs, users have instances. The Elastic IP address is what is used to advertise the data within the instance to the public internet.


Works:

  • Each EIP can be assigned to one instance, in which case it replaces the normal dynamic IP address. Remember, by default, each instance starts with a dynamic IP address.
  • Each instance can have only a single external IP address. It starts out with the default dynamic IP address which can be swapped out for an EIP at any time. If the EIP is designed (or assigned to a different instance) then a fresh dynamic IP is allocated for the instance. The limitation of designating a single IP at a time is due to the way NAT(Network Address Translation) works. Remember that each instance has an internal IP address and an external (public) one, which is translated into the internal one. If two external IPs were translated to the same internal IP then inbound packets would arrive fine, but sorting out outgoing packets (i.e. determining which external IP address to assign to outgoing packets) would be very difficult. Hence, the limitation of a single external IP address per instance at any given point in time.
  • EIPs are free while they are assigned to an instance, but they cost $0.01/hr if they are not assigned. The reason for this charge is the fact that the number of IP addresses worldwide is very limited. Perhaps, in theory, this charge will help prevent users from hogging unused IP addresses that could be dynamically allocated to other users. Yet, in a weird way, there is no additional cost to Amazon for an assigned static IP as opposed to a dynamic IP because while an EIP is assigned to an instance it actually frees up a dynamic IP.
  • Assigning or reassigning an IP to an instance takes a couple of minutes, which is longer than I would have hoped for, but I can imagine that many network devices need to be updated in the infrastructure to make it all happen.

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