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You restart your EC2 instance, come back the next morning, and your SSH command fails. Nothing's broken - your instance's public IP changed. Understanding why is the difference between fighting AWS networking and working with it.

There are three kinds of IP address in play, each with a different job.

The three addresses at a glance

Address	Reachable from	Changes?	Costs?
Private IP	Inside the VPC only	Stays for the instance's life	Free
Public IP	The internet	New one on stop/start	Free (while attached)
Elastic IP	The internet	Fixed - yours until you release it	Free while in use, charged if idle

Private IP - the instance's internal address

Every instance gets a private IP (like 10.0.1.25) from its VPC's address range. This is how resources inside your network talk to each other - your web server to your database, for example. It's:

Only reachable within the VPC (and peered/connected networks).
Stable - it stays the same for the life of the instance, even across stop/start.
Free.

   VPC  10.0.0.0/16
   ┌──────────────────────────────────────────┐
   │  web server          database              │
   │  10.0.1.25  ───────►  10.0.2.40            │
   │      (talk via private IPs, never leave VPC)│
   └──────────────────────────────────────────┘

Because private IPs are stable, internal communication should use them - not public IPs.

Public IP - the instance's internet address

A public IP (like 54.221.10.8) is what makes your instance reachable from the internet. AWS assigns one from its own pool when the instance launches (if the subnet is configured for it).

The catch that bites everyone:

A standard public IP is not permanent. Stop and start the instance and AWS gives it back to the pool - your instance gets a different public IP next time. (A reboot keeps it; a stop/start does not.) Any DNS record, SSH config, or bookmark pointing at the old IP now points at nothing.

   Day 1:  stop/start  →  public IP = 54.221.10.8
   Day 2:  stop/start  →  public IP = 3.108.55.2   ← different!
           your SSH command and DNS now break

For throwaway experiments this is fine. For anything you want to reliably reach, you need a fixed address - which is exactly what an Elastic IP is.

Elastic IP - a public IP that stays put

An Elastic IP (EIP) is a public IPv4 address that you own within your account and can keep as long as you want. You allocate one, then associate it with an instance. It does not change on stop/start, and you can move it from one instance to another.

   Elastic IP  13.232.0.50  (yours, fixed)
        │  associate
        ▼
   ┌──────────┐        instance dies / you replace it
   │ Instance │  ──►   re-associate the SAME EIP to the new one
   └──────────┘        clients never notice - address unchanged

That last property is the real value: if an instance fails, you can point the same EIP at a replacement and everything that knew the old address still works.

The billing gotcha

Elastic IPs are free only while associated with a running instance. AWS charges for an EIP that you've allocated but left unattached (or attached to a stopped instance) - precisely to discourage people from hoarding scarce IPv4 addresses.

If you allocate an Elastic IP and don't attach it - or you stop the instance it's on - you start paying a small hourly charge for the idle address. When you're done with an EIP, release it (not just disassociate). An unreleased, unused EIP is a classic line item on surprise bills.

When do you actually need an Elastic IP?

Often you don't. Reach for the right tool:

Need	Use
A stable public address for a single long-lived server	Elastic IP
A stable address for a fleet behind a load balancer	The load balancer's DNS name (no EIP needed)
A friendly name instead of an IP at all	Route 53 DNS pointing at the resource
Internal service-to-service traffic	Private IP (or internal DNS)

In modern architectures you reach for Elastic IPs less than you'd think. The common pattern is to put servers behind a load balancer and point a Route 53 domain name at it - so individual instances can come and go (and change public IPs freely) without anyone noticing. EIPs shine for single, pet servers that must keep one fixed address.

Quick mental model

Private IP = your house's room number - only meaningful inside the building (VPC).
Public IP = a hotel room AWS assigns you - you might get a different one each stay (stop/start).
Elastic IP = a permanent street address you rent - yours until you give it back, movable between buildings.

With addressing clear, the next pages turn to the instance's disk - EBS - starting with what the root volume actually is.

Private IP vs Public IP vs Elastic IP