My experience with system administrators on all platforms is that the network is something of a mysterious think.  When it all works, it is beautiful.  When it fails.... 

Despite what the elites tell you about the OSI stack, networks have 4 levels: physical (wires, or radio), internet (IP), transport (TCP, UDP, SCTP), application.

I'm going to talk about troubleshooting from the bottom layer up:

1. Diagnosing physical layer problems: ethernets, Wifi, ARP
2. Failure modes in IPv4 and IPv6 (if possible - requires an IPv6 infrastructure, which might not be available).  Routing failures.  Name resolution failures.
3. Transport problems (TCP, UDP)
4. Application problems (HTTP, HTTPS, SSH - depending on how much time I have left)
5. How to prepare for network failures using documentation