DHCP Pool Exhaustion in a Shared School Lab
A K-12 lab has 24 student desktops, 2 teacher laptops, and 6 tablets on one VLAN, 192.168.10.0/24. The DHCP server pool is 192.168.10.100 through 192.168.10.120 inclusive, with 8-hour leases. A printer and a file server use static addresses .10 and .11. On Monday morning, about 12 devices show addresses in 169.254.x.x and cannot reach the internet; the rest work. The devices were left powered on over the weekend, but many had been shut down or asleep. The technician suspects a bad cable, but the pattern is spread across two switches and all affected devices recover after a long wait or a reboot. The school also has guest tablets that join the same VLAN during events. The principal wants a fix that will not require renumbering the static devices.
1. What is the most likely cause of the 169.254.x.x addresses? Support your answer with a calculation and at least two checks the technician should run before changing configuration.
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169.254.x.x is an APIPA address, which a host self-assigns when DHCP fails. The pool .100-.120 inclusive has 21 addresses. The DHCP-capable devices are 24 desktops + 2 laptops + 6 tablets = 32 clients. Even if the two static devices are excluded, 32 > 21, so at least 11 clients cannot obtain a lease; 12 failures is consistent with one additional lease still held by an absent device. The spread across two switches also argues against a single bad cable. Checks: inspect DHCP scope statistics and logs for pool exhaustion, lease duration, and bad-address conflicts; run ipconfig /all or the equivalent on affected and working hosts; confirm the static .10 and .11 are excluded from the pool; and capture DHCP Discover/Offer traffic on the VLAN. Only after confirming should the technician change the scope.
2. Recommend an immediate remediation and a long-term design, including specific subnet or pool changes, lease adjustments, and how to preserve the static printer/server addresses.
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Immediate: expand the pool on 192.168.10.0/24 to, say, .100-.200 (101 addresses), keep .10 and .11 excluded, and reduce lease time to 2-4 hours during events so stale leases recycle faster. Long-term: put student, staff, and guest devices on separate VLANs/subnets; for example, use 192.168.10.0/24 for staff and student devices and 192.168.20.0/24 for guests, each with a DHCP pool sized for peak devices plus 20-30% headroom. Reserve the printer and server by DHCP reservation or keep them static and outside every pool. Use a /23 only if a single larger broadcast domain is acceptable; VLAN separation is better for guest isolation. Monitor pool utilization and alert at 80%. This preserves .10 and .11 and avoids renumbering the static devices.