What a network is and why it is useful, networks by size (LAN, MAN, WAN), common topologies, the devices that connect networks, and the basic vocabulary of the internet and the web - IP addresses, DNS, URLs, HTTP and HTTPS.
Reading the 8 sections above, the worked examples and the practice set is free — the whole knowledge base is on the free plan, along with flashcards, private notes and Ask Onely.