Sixteen Addresses for Twenty Desktops
The computer lab has 20 identical desktops, all set to obtain an IP address automatically. They connect through one 24-port switch to a router at 192.168.1.1. The router's DHCP pool is set to 192.168.1.100–192.168.1.115, with a lease time of one day. Three network printers use static addresses 192.168.1.200–202, and the teacher's laptop also uses DHCP. For the last week, some desktops show an address beginning 169.254 when students run ipconfig, and which machines fail changes every morning. A failing PC can ping another failing PC on the same switch but cannot ping 192.168.1.1; a working PC can ping the router. Link lights are on everywhere. A salesperson advises replacing the switch and reinstalling Windows on the affected PCs. Before spending money, the instructor wants the actual cause identified and a fix that will not create new address conflicts.
1. Interpret 169.254.x.x and explain why the failing set changes daily. Calculate whether the DHCP pool can serve the demand.
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169.254.x.x is a link-local (APIPA) address that a Windows host assigns itself when it receives no DHCP lease. It is not a valid address for reaching the router, so the PC can talk to other link-local machines on the same switch but has no route to 192.168.1.1. The link lights and same-switch pings show that the cabling and switch ports are working; this is a configuration or capacity problem, not a hardware failure.
The pool 100–115 inclusive contains 16 addresses: 115 − 100 + 1. DHCP demand is 20 desktops plus the teacher's laptop, at least 21 clients; the printers are static and do not consume leases. Since 21 > 16, at least five clients cannot get an address at any one time.
Leases last one day, and machines that already hold a lease keep it. As PCs are shut down, restarted or their leases expire, the set of machines holding the 16 addresses changes, which is why a different group fails each morning. The diagnosis is DHCP pool exhaustion.
2. Give an ordered set of checks to confirm the cause and the correct fix. Why is assigning static addresses to all 20 PCs a poor solution?
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Order: (1) run ipconfig /all on a working and a failing PC and compare DHCP server, lease obtained and lease expires fields; (2) run ipconfig /release followed by ipconfig /renew on a failing PC; (3) inspect the router's DHCP client/lease table to see whether all 16 addresses are allocated; (4) ping 192.168.1.1 from a working PC to confirm the router is reachable; (5) check the switch uplink only after the address evidence, since link lights already suggest it is fine.
Fix: enlarge the pool, for example 192.168.1.100–192.168.1.180, which leaves the printers at 200–202 outside the range and allows phones and visitors. Keep DHCP, add reservations for the printers if desired, and document the range. A /24 subnet has ample space.
Static addresses on all 20 PCs would work temporarily but invites duplicate-address conflicts, makes every change manual, prevents students' devices from connecting easily, and does not solve the underlying capacity planning. If static addressing is used at all, it must be documented and excluded from the DHCP pool.
3. A colleague suggests reducing the lease time to two hours. Evaluate the suggestion.
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Lease time controls how long a client may keep an address; it does not create addresses. With 21 DHCP clients and 16 addresses, all 21 can be present at the same time during a practical class, so the pool is still exhausted whatever the lease time. Shorter leases only recycle addresses faster when devices leave, and they increase renewal traffic and the chance of disruption during a lesson.
A shorter lease could reduce the problem if many devices were transient and left quickly, so it may help at the margin, but it cannot replace a larger pool. The correct fix is to expand the pool or the subnet, with the printers kept outside the dynamic range.
Judgement: identify the binding constraint. The constraint is the number of simultaneous clients, not the duration of the lease. The student should say this clearly and propose the capacity change, then mention lease tuning only as a secondary measure.