Just to throw a random thought out there... given that in today's world almost every system we build is a "distributed system" to some extent, and given that I've found that quite a few developers don't know a lot about underlying network fundamentals, I'd suggest - depending on your current knowledge levels - spending some time boning up on networking. I mean things like learning how DNS works, what ARP is, how IP addressing works (mostly still IPv4, but don't ignore IPv6 altogether), and similar. In that same vein, learn to use common network troubleshooting tools well, from ping, to traceroute, to netstat, netcat, tcpdump / wireshark, and even nmap.