My First Useful Wireshark Capture: ARP Before ICMP
Capturing a simple ping showed that the interesting packet often arrives before ICMP: ARP has to resolve the Layer 2 destination first.
ICMP
Ping Works but the Browser Does Not: Separating IP, DNS and Applications
Connectivity tests become useful only when each test is tied to a layer: local addressing, routing, DNS resolution, TCP reachability and the application itself.