The Lab That Has Internet but No Web
Greenfield School has a 30-PC lab connected to a 100 Mbps fibre line through a router at 192.168.1.1. The router's DHCP pool is 192.168.1.100-192.168.1.150, the network printer is manually set to 192.168.1.50, and the library server is 192.168.1.10. A teacher's laptop was also configured manually with 192.168.1.50 for a camp and is now back on the lab network. On Monday, 8 of the 30 PCs show 'connected, no internet'; two show an address beginning 169.254; the teacher's PC can ping 8.8.8.8 but the browser reports 'DNS address could not be found'. The lab switch is a 100 Mbps 24-port switch, and a second 8-port switch is daisy-chained to it for the last 6 PCs. The Wi-Fi access point is in the corridor, 25 m from the lab, and video lessons freeze every few minutes. The principal asks why 'the internet is down' when the fibre provider says the line is fine.
1. Diagnose the two-PC 169.254 symptom and the duplicate 192.168.1.50 problem. Propose specific fixes, including whether the DHCP pool is large enough for 30 PCs.
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A 169.254.x.x address is an APIPA address, which means the PC did not receive a DHCP lease. The pool 192.168.1.100-192.168.1.150 has 51 addresses, which is more than enough for 30 PCs, so pool exhaustion is not the explanation. The cause is more likely a link or uplink problem, a disabled DHCP service on the router, or a cable/switch port failure affecting those two PCs.
The duplicate 192.168.1.50 is an address conflict: the printer and the teacher's laptop both claim it. Symptoms can include intermittent connectivity, ARP warnings, and 'connected, no internet' on some machines. Fix by setting the teacher's laptop back to DHCP, reserving 192.168.1.50 for the printer on the router or excluding it from the DHCP pool, and keeping the printer outside the dynamic range. Then renew leases on affected PCs and check the daisy-chained switch uplink. A quick test is ipconfig /all on a failing PC to confirm whether it has a valid lease and correct gateway.
2. Explain why ping 8.8.8.8 succeeds while the browser reports a DNS error. Give tests that distinguish DNS from a line outage, and state a reasonable fix.
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Ping to 8.8.8.8 uses an IP address, so it tests routing and the fibre line without using DNS. Success proves the line and default route are working. The browser failure is therefore a name-resolution problem, not a total internet outage. The browser cannot turn a domain name into an IP address.
Tests: nslookup example.com or ping example.com to see whether DNS resolves; ipconfig /all to check the configured DNS server; try opening a site by IP address; and compare with a public DNS server such as 1.1.1.1. If nslookup fails but IP ping works, DNS is the fault. Fix by correcting the DNS server setting on the router or on the PC, for example using the router as DNS forwarder or a reliable public resolver. Also flush the DNS cache and renew the lease. The fibre provider is correct that the line is up.
3. Video lessons freeze every few minutes. Identify the likely bottlenecks in the switch layout and Wi-Fi path, and recommend changes with justification.
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The last 6 PCs share a daisy-chained 8-port switch uplinked to a 100 Mbps 24-port switch. That uplink is a shared 100 Mbps path for those PCs, and the whole lab also shares a 100 Mbps WAN link. Video is delay-sensitive, and a saturated uplink causes buffering. The Wi-Fi access point is 25 m away in a corridor, so the lab likely receives weak or interfering 2.4 GHz signal; walls and other classes add contention. Wi-Fi is half-duplex, so 30 devices sharing one AP can freeze even when the fibre line is fine.
Recommend a Gigabit switch for the lab and a Gigabit uplink for the second switch, with wired connections for fixed PCs. Move or add an access point inside the lab, use 5 GHz where possible, and limit video streaming to wired stations. If the WAN link is only 100 Mbps, prioritize video traffic or reduce simultaneous streams. Check switch port errors and duplex mismatches before buying hardware.