A 25-Node Lab Sharing One Broadband Line
A school computer lab has 25 desktop computers, one network laser printer and one broadband router with four LAN ports. Two 16-port unmanaged switches, A and B, are used. Switch A connects to the router, the printer and 10 desktops. Switch B connects to Switch A through one cable and to 15 desktops. All 25 desktops have manually configured addresses in 192.168.1.0/24. The router's DHCP server is switched off because 'static addresses are safer'. A teacher connects a laptop to a free port on Switch B, sets it to obtain an address automatically, and gets 169.254.18.7. The laptop can neither browse nor print. The desktops continue to work, but internet speed drops sharply when all 25 are online. The school wants to add visitor Wi-Fi later without renumbering the existing lab.
1. Explain the address 169.254.18.7 and why the desktops still work. Give two likely causes and the first diagnostic step.
Read the worked answer
169.254.x.x is a link-local or APIPA address. Windows assigns it when the machine is set to obtain an address automatically but no DHCP server responds. It allows communication only on the local link and is not a usable lab address, so the laptop cannot route to the internet or reach the printer by its normal address.
The desktops still work because they use manually configured static addresses; they never ask for DHCP. Their success therefore hides the fact that DHCP is unavailable.
Two likely causes are: the router's DHCP server is switched off, as stated; or the laptop's switch port or cable is faulty, so its DHCP request never reaches the router. A less likely cause is a DHCP pool that has no free addresses, but with only one DHCP client that is unlikely here.
The first check is to run ipconfig /all on the laptop to confirm that DHCP is enabled and that the address is APIPA. Then check the link light and try another known-good port. If the link is good, inspect the router's DHCP settings and renew the lease.
2. Recommend an addressing plan that keeps 192.168.1.0/24, reserves fixed addresses for the printer, and provides DHCP for visitors. Explain why the printer should be outside the DHCP pool and why the pool should not cover the whole subnet.
Read the worked answer
Use 192.168.1.0/24 with subnet mask 255.255.255.0, giving usable addresses 192.168.1.1 to 192.168.1.254. A workable plan:
- Router: 192.168.1.1 (fixed).
- Printer: 192.168.1.10 (fixed, outside DHCP).
- Existing lab desktops: keep their current static addresses, but document them; for example 192.168.1.50–192.168.1.99.
- Visitor DHCP pool: 192.168.1.150–192.168.1.200.
- Reserve the rest for future static devices and growth.
The printer must be outside the DHCP pool so that its address is never leased to a visitor. A permanent print queue can then point to 192.168.1.10 without the address changing. The pool should not cover the whole subnet because it would eventually hand out addresses already used by the router, printer or static desktops, causing duplicate-address conflicts. The unmanaged switches do not need IP addresses; only managed switches would.
3. Identify the topology used and explain why internet speed drops when all 25 computers are online. Suggest one cabling or switch change and one router setting that would reduce the problem.
Read the worked answer
Each switch forms a star with its own computers, but the two switches are connected in a chain: B → A → router. This is an extended star or daisy-chained star. All traffic from the 15 computers on B, including internet traffic, must cross the single B–A uplink and then the single router uplink. When all 25 computers are active, those shared links become bottlenecks, even though each individual computer has a dedicated switch port.
A practical change is to connect Switch B directly to a second LAN port on the router, so B's traffic no longer passes through A, or replace both switches with one 24-port or 48-port switch to remove the inter-switch hop. Using Gigabit uplinks would also help if the current uplinks are 100 Mbps.
On the router, enable QoS and prioritise interactive classroom traffic over bulk downloads. If visitor Wi-Fi is added later, give it a separate SSID and a bandwidth limit so guests cannot consume the whole broadband line.