Three faults in one school computer lab
A school computer lab has 24 student PCs, one teacher PC, one network printer and a Wi-Fi access point for students' phones, all on the subnet 192.168.1.0/24. The router's DHCP server is configured with a lease time of 8 days and a pool running from 192.168.1.100 to 192.168.1.119. The printer was given the fixed address 192.168.1.105 by hand two years ago. On Monday morning, PCs 21 to 24 cannot reach the internet or the file server and show addresses beginning 169.254. The printer reports an address conflict and is unreachable from some PCs but not others. Separately, whenever the wall socket beside the door is in use, the whole lab becomes extremely slow for everyone, and the switch's port lights flash at full rate even during break when nobody is logged on.
1. The printer's address conflict is a separate fault. Explain why the conflict occurs and describe a fix that keeps the printer's address permanent.
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The printer's address, 192.168.1.105, was set by hand - but 105 lies inside the DHCP pool 100–119. The router knows nothing about the manual assignment, so at some point it hands 192.168.1.105 to a laptop. Two devices then own the same address on the same subnet. Traffic intended for one is delivered to whichever machine answers the ARP request first, which is why the printer works from some PCs and not others, and why the printer itself reports a duplicate-address warning.
The fix must give the printer a stable address that the router will never hand out. Two acceptable solutions:
- Static address outside the pool - for example 192.168.1.20 or .250, with the subnet mask 255.255.255.0 and gateway 192.168.1.1 set manually on the printer.
- DHCP reservation - keep the printer on DHCP but bind its MAC address to a fixed address in the router's reservation table, so it always receives the same IP and the router never leases that address to anyone else.
Option 1 is simpler for a single printer; option 2 scales better when there are several shared devices. Either way, the underlying rule is that fixed addresses must sit outside the dynamic range, not inside it.
2. Explain the slowness that appears when the socket beside the door is used, and the lights flashing at break time. Rank the three faults in the order you would tackle them and justify the order.
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A cable running from one wall socket back into another creates a physical loop between two ports of the same switch. Ethernet frames have no time-to-live field, and switches forward broadcasts out of every other port. A broadcast frame therefore circulates through the loop, is copied each time, and multiplies within seconds - a broadcast storm. The switch's backplane and CPU are saturated forwarding the same frames, so every user on the lab experiences very slow service, and all port lights flash at maximum even with nobody logged on, because the traffic is the storm, not user data.
Order of work, and why:
- The loop first. It degrades every device on the lab at once and can take down the network completely. First action: trace the socket beside the door and unplug the patch lead that returns into a second socket. As a permanent safeguard, enable Spanning Tree Protocol on the switch, which detects the redundant path and blocks one port automatically.
- DHCP exhaustion second. Four machines are completely offline; this is a total loss of service for those users and is fixed in minutes by widening the pool (for example 192.168.1.100–180) and shortening the lease to 8 hours.
- The printer third. It is intermittent rather than dead, affects one device, and is fixed by moving it outside the pool.
Ranking by blast radius - how many users are affected and how completely - is what makes the order correct rather than arbitrary.