I did AS/400 also. Cool stuff. We had token ring and it turns out if you are fast enough you can add in a new node to the existing ring without breaking any ongoing connections or anything. Lay all the cables out, practice the unplug and replay mentally and then boom, new employee online.

But yeah tcpdump, netcat, dig invaluable. Just the knowledge of the TCP handshake and what various timings for “connect failed” might mean about a firewall in the way vs. app not running etc is super useful, cloud or not.