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
https://www.plixer.com/blog/network-layers-explained/
iproute2 utility and IPUP and IPDOWN
http://www.policyrouting.org/iproute2.doc.html#ss9.9
iproute2 utility faq
https://wiki.linuxfoundation.org/networking/iproute2
IPv4 Subnet Calculator
https://www.iplocation.net/subnet-calculator
This is the bomb. I love the detailed output. Choose whatever info you need. Screenshot below.
Broken Link Sniffer
https://www.brokenlinkcheck.com/broken-links.php
Free. Simple. Scans internal and outbound (external facing) hyperlinks.
OpenVPN Community Downloads
https://openvpn.net/community-downloads/
Latest mainstream version, current gen OpenVPN. Still 2.x as of August 2019. Verison 3.x remains experimental. ;)