A collection of excellent online resources from other authors, in no particular order.
All links below should open in a separate browser tab or window
Network layers explained with examples
iproute2 utility and IPUP and IPDOWN
iproute2 utility faq
IPv4 Subnet Calculator
This is the bomb. I love the detailed output. Choose whatever info you need. Screenshot below.
Broken Link Sniffer
Free. Simple. Scans internal and outbound (external facing) hyperlinks.
OpenVPN Community Downloads
Latest mainstream version, current gen OpenVPN. Still 2.x as of August 2019. Verison 3.x remains experimental. ;)