Intermittent APIPA, an IP Conflict and a DNS Failure in a School Lab
A school computer lab has 12 desktop PCs, one network printer and a Wi-Fi router that also acts as the DHCP server. The router's pool is 192.168.1.100 to 192.168.1.150; the printer is manually set to 192.168.1.55 and the router itself is 192.168.1.1. Over a week, PC-07 sometimes boots with the address 169.254.11.23 and cannot reach the printer. A teacher's laptop, configured at home with the static address 192.168.1.55, is now used in the lab, and the printer stops responding whenever the laptop is switched on. On three other PCs, users can ping 8.8.8.8 successfully but the browser reports that no server is found for every website. The lab technician wants to reset the router to factory defaults and reinstall Windows on all PCs. The principal asks for a written diagnosis before approving any work, because the lab is needed for an online examination next week.
1. Explain what 169.254.11.23 means on PC-07 and give an ordered diagnostic sequence before any hardware is replaced.
Read the worked answer
169.254.x.x is an APIPA address, self-assigned when the PC's DHCP request receives no valid offer. It proves the PC has link but no DHCP lease, not that Windows is corrupted.
Order of checks:
- Confirm physical link: switch port LED, cable latch, and swap PC-07's patch cable and port with a known-good PC.
- Check the PC's adapter settings: it must be set to obtain an IP address automatically, with no leftover static address or proxy.
- Check the router's DHCP server status, pool range and lease table, and whether the pool has been exhausted by phones or guest devices. With 51 addresses and 12 PCs this is unlikely, but verify.
- Look for a duplicate address or a rogue DHCP server, and check the router log for the PC's MAC address.
- Run ipconfig /release and /renew and confirm the lease.
The intermittent nature points to a cable, port or connector fault before anything else; replace hardware only after these tests.
2. Diagnose the printer problem and the browser symptom, and decide whether the technician's reset-and-reinstall plan is justified.
Read the worked answer
The printer fault is an address conflict. The printer is static at 192.168.1.55, and the teacher's laptop is also static at 192.168.1.55. When both are on, ARP replies collide and the printer becomes unreachable or intermittent. The fix is to keep the printer at .55 and change the laptop to DHCP, or move the printer outside the DHCP pool and reserve its address on the router.
The browser symptom is a name-resolution failure, not a connectivity failure: pinging 8.8.8.8 proves IP routing works, while 'server not found' points to DNS. Check ipconfig /all on the affected PCs, compare the DNS server with the router's address, test with nslookup, and correct any wrong static DNS entry or disabled DNS relay on the router.
The reset-and-reinstall plan is not justified. It would erase the router configuration and risk the exam schedule without addressing the documented causes. Approve targeted changes, tested one at a time.
3. Recommend configuration and documentation changes that would prevent these faults from returning, including what to do before the online examination.
Read the worked answer
- Reserve addresses: keep infrastructure (router, printer, switches) on static addresses outside the DHCP pool, and document them in an IP register.
- Keep the DHCP pool for clients only, and extend or monitor it if guest devices are allowed; consider a separate guest SSID with client isolation.
- Standardise all lab PCs on DHCP, and remove static addresses from portable laptops or give them a documented reservation.
- Label cables and ports, keep a simple test log, and store a known-good router configuration backup.
- Before the exam, test every PC with ipconfig, a ping to the gateway and a DNS lookup, verify the printer from two PCs, and run a full mock session.
- Tell users not to change network settings, and keep the router admin password controlled.
Record the root causes in the lab log so the next technician does not repeat the reset-and-reinstall approach.