Networking
Cisco's Sandbox. Allows free access to a wide variety of Cisco technologies for development purposes.
https://developer.cisco.com/site/sandbox/
and the Cisco networking academy
https://www.netacad.com/courses/networking
Network+ training
https://www.professormesser.com/
Security Onion is a fully featured open source network security monitoring platform. It's a fantastic way to learn the practice of NSM.
https://github.com/Security-Onion-Solutions/securityonion
https://docs.securityonion.net/
A curated constantly-growing library of infographics for network students
https://github.com/DelfinoRT/Networking-Study-Stuff
IANA's official mapping of ports and services
https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xml
All the packetlife cheat sheets are useful.
https://packetlife.net/library/cheat-sheets/
How IPSEC works
https://networklessons.com/cisco/ccie-routing-switching/ipsec-internet-protocol-security
Packet Captures (PCAP)
Legendary site with malware traffic samples. Very useful for practicing analysis.
https://www.malware-traffic-analysis.net/
Chris Sanders of the excellent Practical Packet Analysis book has a library of PCAP
https://github.com/chrissanders/packets
The Zeek (formerly Bro) project has an extensive library
https://github.com/zeek/zeek/tree/master/testing/btest/Traces
As does the Wireshark project itself
https://wiki.wireshark.org/SampleCaptures
Benign PCAP samples
http://njrusmc.net/jobaid/jobaid.html
Learn Wireshark with sample PCAP's
DNS
Learn how DNS works. Or indeed doesn't work. "It's not DNS. There is a no way it's DNS. It was DNS"