Before you can build reliable protocols or cloud-scale VPCs, you must understand the bedrock of the internet: IP addresses, physical routing, and logical ports.
IPv4 vs IPv6, binary structure, subnet masks, and how routers determine where a packet goes next.
How an operating system multiplexes thousands of logical connections over a single physical wire using ephemeral ports.
How billions of devices share a handful of public IPs. Dissect RFC 1918 and the state tables of Network Address Translation.