Phase Verbs / Action Phrases
| Phrase | Meaning |
|---|---|
| attach (an ENI) | connect a network interface to an EC2 instance |
| detach / move (an ENI) | remove an ENI from one instance and connect to another |
| bind (to an AZ) | ENIs are locked to the AZ where they were created |
| represent (a virtual network card) | ENI is the logical equivalent of a physical network card |
| provide (network connectivity) | ENI is what gives an EC2 instance access to the network |
Technical Vocabulary
| Term | Definition |
|---|---|
| ENI (Elastic Network Interface) | A virtual network card in a VPC; the networking component of an EC2 instance |
| eth0 | The primary (default) ENI attached to an EC2 instance |
| eth1 | A secondary ENI you add to an EC2 instance for additional network interfaces |
| Primary private IPv4 | The main private IP on an ENI; cannot be changed after assignment |
| Secondary IPv4 | Additional private IPs you can add to an ENI |
| Elastic IPv4 | A static public IP that can be attached to an ENI (one per private IPv4) |
| MAC address | Hardware identifier attached to an ENI |
| VPC | Virtual Private Cloud — the isolated network where ENIs live |
Key Concepts
What is an ENI?
- A logical component in a VPC representing a virtual network card
- Gives EC2 instances their network connectivity (IP addresses, security groups)
- Each EC2 instance comes with at least one ENI (eth0) by default
- ENIs can be created independently from EC2 instances
ENI Attributes
Each ENI can have:
- Primary private IPv4 (always)
- One or more secondary IPv4 addresses
- One Elastic (public) IPv4 per private IPv4
- One or more public IPv4 addresses
- One or more Security Groups
- A MAC address
ENI Boundary: AZ-Scoped
- An ENI is bound to a specific Availability Zone
- You can only attach an ENI to EC2 instances in the same AZ
- Cannot move an ENI across AZs
Moving ENIs for Failover
Instance A (eth0, eth1) → eth1 can be moved to → Instance B- You can move a secondary ENI (eth1) from one instance to another in the same AZ
- The private IP moves with the ENI — Instance B now has that IP
- Useful for: failover scenarios where another instance takes over a known private IP
Common Use Case: IP Failover
- Instance A has a static private IP on eth1
- If Instance A fails, detach eth1 and attach it to Instance B
- Instance B now answers on the same private IP — failover without reconfiguration
Exam Tips
- ENI = virtual network card in a VPC; carries IPs and security groups
- Bounded to one AZ — cannot move across AZs
- Can be attached/detached from EC2 instances on the fly
- Moving an ENI moves its private IP → useful for network-level failover
- Each EC2 instance always has at least one ENI (eth0 = primary)