Diagnosing a Lab Network Failure Without a Mass Reinstall
A school computer lab has 12 desktop computers connected to one 16-port unmanaged switch. The switch uplinks to the router, which also runs DHCP. A shared folder named Staff_Notes is on the teacher's PC at 192.168.1.10. On Monday, six student PCs work normally. PC-07 shows a link light but cannot open the shared folder; its address is 169.254.18.5. PC-09 can ping the router but not the teacher's PC. PC-11 has been given static address 192.168.1.100, the same as the network printer, and users report intermittent printing. The lab technician wants to reset every PC and reinstall the network card driver, which will take the whole practical period. The teacher has a 40-minute class and needs the shared folder for an activity. No spare switch or router is available. The lab has a working cable tester and one known-good patch cable.
1. What should the technician check first, and why is a mass reset of all 12 PCs a poor decision?
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Start at the physical/link layer, then addressing, then name/service access, then the teacher PC. Do not reset all 12 PCs: six work, so the common switch, router and DHCP are at least partly functional, and a mass reinstall uses the only 40-minute class without evidence. First use the known-good patch cable and tester on PC-07 and PC-11; confirm link speed and switch port. Then run ipconfig /all on PC-07, PC-09 and PC-11. PC-07's 169.254 address points to DHCP failure on that client or its cable/port, not to the teacher's share. PC-09 can reach the router but not the teacher PC, so check the teacher PC's firewall, file-sharing settings and whether it is on the same subnet. PC-11's duplicate address explains intermittent printing; change it to a free address outside the DHCP pool or reserve it. Only after these checks consider a driver reinstall on the one faulty PC.
2. Explain what the 169.254.18.5 address means, list likely causes on PC-07, and describe how to isolate the cause.
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A 169.254.x.x address is an APIPA/link-local address. The PC gives itself this address when it sends DHCP Discover messages but receives no DHCP Offer. It can usually communicate only on the local link and cannot route properly, so it will not reach 192.168.1.10 through the router. Likely causes on PC-07: bad patch cable or switch port, DHCP scope exhausted, DHCP service stopped, or a static/alternate configuration left on the adapter. The link light only proves physical link, not that DHCP works. Test the same cable/port with a known-good laptop; if that laptop gets a 192.168.1.x lease, the fault is in PC-07's adapter configuration or driver. If not, move to a working port and check the router's DHCP pool. Do not 'fix' it by assigning a random static address without checking the range.
3. How should PC-11's duplicate address be fixed and prevented from recurring?
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Change PC-11 from 192.168.1.100 to an address outside the DHCP pool, for example 192.168.1.200 after confirming it is unused, or create a DHCP reservation. Then test printing from PC-11 and another PC. Prevention: keep an IP address register with device name, MAC address, assigned address and whether it is static or DHCP; use reservations for printers and the teacher PC; label switch ports; and set the DHCP pool so it excludes static addresses. A duplicate IP causes intermittent failures because ARP replies alternate between two devices, so the printer and PC-11 may each receive traffic meant for the other. Document the change, because the next technician needs to know why the address changed.