Intermittent Lab Network After a Switch Upgrade
A 24-PC computer lab is connected to an ISP router at 192.168.1.1/24. The router’s DHCP pool is 192.168.1.100–192.168.1.199. Ten staff PCs use static addresses 192.168.1.10–192.168.1.19, and a network printer uses 192.168.1.50. A new 24-port unmanaged switch replaced a failed 16-port switch. The uplink from the new switch to the router is a 3 m patch cord. One PC in an adjacent room is connected through a 3 m patch cord, a 35 m factory cable, a coupler, a 65 m factory cable and another 3 m patch cord. That PC intermittently receives a 169.254.x.x address, while a teacher’s laptop is told there is an IP conflict for 192.168.1.100. All other PCs work, but throughput drops when large files are copied. No cable tester is available, but there is one spare 15 m patch cord.
1. Diagnose the 169.254 address and the IP conflict. Show the calculations and the checks you would perform before changing any settings.
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A 169.254.x.x address is APIPA: the PC sent DHCP Discover but received no offer. The adjacent-room path is 3 + 35 + 65 + 3 = 106 m, plus a coupler and connectors. Copper Ethernet is limited to 100 m for a channel, so this link is over the limit and the coupler adds loss and reflection. That explains intermittent link and DHCP failure. I would check the switch port LED, swap the two patch cords, temporarily move the PC next to the switch with the 15 m patch cord, and inspect the coupler and cable for damage. If it works short, the permanent run is the fault.
The IP conflict for 192.168.1.100 is separate: .100 is the first address in the DHCP pool, so either a device has been manually set to .100, a stale reservation exists, or a second DHCP server is present. Before changing the laptop, run ipconfig /all, ping .100, check arp -a, and inspect the router’s lease table. Disconnect suspected devices one at a time and watch which MAC address answers. Changing the laptop to another static address would only hide the duplicate.
2. Propose a clean addressing plan for 24 student PCs, 10 staff PCs, the printer and four future access points, including DHCP ranges and exclusions.
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Use the existing 192.168.1.0/24 network but document it. Reserve 192.168.1.1 for the router, .2–.9 for switches and access points, .10–.49 for static staff PCs and the printer, .50–.99 for servers and future infrastructure, and .100–.199 for DHCP. Keep .200–.254 unused for growth. Exclude .1–.99 from the DHCP pool on the router so the router never hands out an address already used statically. Put the printer on a DHCP reservation if it is easier to manage, but keep its address outside the dynamic range. The 24 student PCs can lease from .100–.123, leaving room for laptops and phones.
Document the plan in a simple table: device name, MAC address, address, assignment method and location. Label the printer and access points. After the change, renew leases on all PCs and verify that no device answers on a static address that is also leased. The plan should leave the 169.254 problem behind because the adjacent-room link will be fixed, not because the address range was changed.
3. The lab needs the adjacent-room PC reliable before online exams. Compare fixing the cable, adding a small switch in the adjacent room, or using Wi-Fi, and recommend one with justification.
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Fixing the cable is the most reliable option: replace the 35 m + coupler + 65 m path with one certified Cat6 run under 90 m, including patch cords. This removes the over-length channel and the coupler, gives full duplex and is easiest to certify. Adding a small switch in the adjacent room can work if the first segment from the main switch is under 100 m and the second segment from the new switch to the PC is also under 100 m. It adds a power point and one more device but is acceptable when pulling a new cable is impossible.
Wi-Fi is the weakest choice for online exams: it is a shared medium, subject to interference and variable latency, and the room may have poor coverage. It should be a temporary backup only. My recommendation is a single new Cat6 run, tested end to end. If the building cannot be disturbed before exams, install a small unmanaged switch at the junction with a short, tested uplink and a short patch to the PC, then schedule the permanent cable. In both cases, label the ports and record the link speed and error counters after the change.