A new desktop on the wrong subnet
A coaching centre has one broadband router at 192.168.1.1/24, a 24-port switch and a Wi-Fi access point in the same room. The centre owns 14 desktop PCs, 6 laptops, 3 network printers and 8 smartphones. The router runs DHCP with a pool of 192.168.1.100-192.168.1.120, and the printers were manually given 192.168.1.50, .51 and .52 by a former technician. A newly installed desktop is manually set to 192.168.2.150 with mask 255.255.255.0 and gateway 192.168.1.1, but it cannot reach the printer at 192.168.1.50 or the internet. Two laptops show addresses in 169.254.10.x. No VLANs are configured. The centre wants all devices to work, the printer addresses to stay stable, and future address conflicts to be avoided without buying a new router. The switch is unmanaged.
1. Diagnose why 192.168.2.150 cannot reach 192.168.1.50 or the internet, and give the minimal correction.
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With mask 255.255.255.0, the host at 192.168.2.150 is on network 192.168.2.0/24. The printer 192.168.1.50 and the gateway 192.168.1.1 are on 192.168.1.0/24, so they are off-subnet. The host will try to send their traffic to its default gateway, but its configured gateway 192.168.1.1 is itself off-subnet and cannot be reached by ARP on the local link. Result: no printer access and no internet. The minimal correction is to give the desktop an unused address in 192.168.1.0/24, for example 192.168.1.151 if that address is outside the DHCP range and not reserved, with mask 255.255.255.0 and gateway 192.168.1.1. Alternatively, add a DHCP reservation and let it obtain the address automatically.
2. Explain the 169.254.10.x addresses on the two laptops and relate them to the DHCP pool size.
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169.254.x.x is an APIPA address, assigned by Windows when a DHCP server cannot be reached or does not offer a lease. The pool 192.168.1.100-192.168.1.120 has only 21 addresses, but the centre has 14 + 6 + 3 + 8 = 31 devices, and the three printers are outside the pool. Once the pool is exhausted, additional laptops fail to obtain a lease and self-assign APIPA addresses, which are not routable to the 192.168.1.0/24 network. The fix is to enlarge the pool, for example 192.168.1.100-192.168.1.199, and to keep the printers on reserved addresses outside it. Checking the router's DHCP lease table and whether the Wi-Fi clients are on the same Layer 2 network would confirm the diagnosis.
3. Propose an addressing plan that keeps printer addresses stable and avoids conflicts. Include the DHCP range and reservations.
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Keep the network 192.168.1.0/24 with mask 255.255.255.0 and gateway 192.168.1.1. Reserve 192.168.1.50-192.168.1.52 for the three printers, either as static addresses with matching documentation or as DHCP reservations tied to their MAC addresses; reservations are easier to manage and prevent duplicates. Exclude the reserved block from the dynamic pool. Set the DHCP pool to 192.168.1.100-192.168.1.199, which gives 100 leases for about 28 client devices plus room for guests. Keep servers and network equipment below .100. Record the plan, and check for conflicts before assigning any static address. This keeps printer addresses stable, gives every device a lease, and avoids the overlapping 192.168.2.0/24 mistake.