A structured lesson workspace with readable content, hands-on examples, and a clean path to completion.
A user reports they can't reach the file server, but everything else works. `ping` to the server fails, but `ping` to the gateway succeeds. The user hasn't changed anything since yesterday. You're on the phone with them and need to diagnose this in under five minutes.
IPv4 addressing faults come in a small number of flavors. Learn to recognize each from host-side output and you'll resolve these calls faster than walking them through the GUI.
Host says `10.20.30.45/25`. Under /25, the subnet is 10.20.30.0 – 10.20.30.127 with usable hosts 10.20.30.1 – 10.20.30.126. Now check the gateway:
Gateway is `10.20.30.1`. Under /25 that address is inside the same subnet — good. Ping it.
Gateway is fine. Now the file server, which lives on a different subnet. If ping to the server fails but gateway succeeds, the fault is upstream — either routing, ACL, or the server itself. This is where you escalate or log into the router.
Mask mismatch is the sneakiest fault. If the user's host has /25 but the network is actually /24, the host thinks the local subnet ends at .127. A server at `10.20.30.200` looks remote — so the host sends traffic to the gateway instead of ARPing directly for the server. If the gateway is on the /24 side, the traffic may actually work; if the gateway also has /25, it breaks.
The telltale sign: `arp -a` shows inconsistent entries. Some hosts on the same subnet resolve directly, others route via the gateway when they shouldn't. If the mask is wrong, comparing ARP tables across affected and healthy hosts reveals it in seconds.
Two different MACs answering for the same IP — classic duplicate IP. Find both hosts and either shut one down or renumber. On managed switches, `show ip arp` on the gateway shows the same IP with alternating MACs.
Section 3 complete. You can convert, subnet, allocate with VLSM, plan real address schemes, and diagnose IPv4 faults. Section 4 pivots to IPv6 — where many of these skills transfer but the notation and defaults differ.
Finish the lesson once you have worked through the material. This awards ★ 40 XP.