1.7 Given a scenario, use appropriate IPv4 network addressing
Address Types
📘CompTIA Network+ (N10-009)
Definition:
APIPA is a feature in Windows (and some other operating systems) that automatically assigns an IP address to a device when it cannot get an IP address from a DHCP server. Essentially, it’s a “backup plan” that allows a device to communicate on a local network even if the DHCP server is unavailable.
How APIPA Works
- DHCP Attempt:
- When a device connects to a network, it usually asks a DHCP server for an IP address.
- DHCP assigns an IP automatically (like 192.168.1.10) along with a subnet mask, default gateway, and sometimes DNS servers.
- DHCP Failure:
- If the DHCP server doesn’t respond (maybe it’s down or unreachable), the device cannot get an IP address automatically.
- APIPA Activation:
- The device then assigns itself an IP address in the range 169.254.0.1 to 169.254.255.254.
- Subnet mask: 255.255.0.0 (This allows devices in the 169.254.x.x range to communicate directly with each other on the same local network).
- Network Communication:
- Devices using APIPA can communicate only with other devices that also have APIPA addresses on the same local network.
- They cannot access devices outside the local network, such as the internet, because there’s no default gateway.
Key Facts About APIPA
| Feature | Detail |
|---|---|
| IP Range | 169.254.0.1 – 169.254.255.254 |
| Subnet Mask | 255.255.0.0 |
| DHCP Required? | No, APIPA works only when DHCP fails |
| Communication | Local network only, no internet |
| Operating Systems | Windows supports APIPA natively; some Linux/Unix systems have similar features (like Zeroconf) |
Why APIPA Exists in IT Environments
- Maintains Local Network Communication:
Even if DHCP is down, computers on the same LAN can still communicate, share files, and print to local printers. - Helps Troubleshoot Network Issues:
If a device has a 169.254.x.x IP, it’s a clear sign that the DHCP server is unavailable, which helps IT staff quickly identify the problem. - Automatic Recovery:
Once the DHCP server comes back online, devices with APIPA will attempt to request a proper IP address again automatically.
Important Exam Tips
- APIPA = 169.254.x.x addresses.
- If you see this, think “DHCP failure.”
- APIPA devices cannot access the internet.
- They can only talk to other devices with 169.254.x.x IPs.
- Subnet mask is always 255.255.0.0.
- APIPA is automatic; users do not configure it manually.
- Common scenario for CompTIA questions:
- A Windows device cannot connect to the network and shows a 169.254.x.x IP.
- The correct explanation: DHCP server is not reachable → APIPA assigned an address.
Example Scenario in an IT Environment
- A small office has a DHCP server at 192.168.1.1.
- A computer is turned on, but the DHCP server is temporarily offline.
- The computer cannot get an IP from DHCP.
- Windows assigns the computer 169.254.12.45 automatically (APIPA).
- The computer can still communicate with other computers on the same subnet using APIPA (like 169.254.8.20) but cannot access the internet.
- When the DHCP server comes back online, the computer requests a proper DHCP IP (like 192.168.1.25) and now can access the internet.
✅ In short: APIPA is like a “self-assigned local IP” when DHCP fails. It keeps local communication alive but cannot reach the internet. Seeing 169.254.x.x is a classic troubleshooting clue on the exam.
