The OSI model, end-to-end principle, and sockets.
Ethernet, MAC addresses, CSMA/CD, and switching. The actual wires.
RIP, count-to-infinity, and poisoned reverse.
OSPF and flooding the network with Dijkstra updates.
CIDR blocks, subnets, and longest prefix matching.
Ports, checksums, and the three-way handshake.
Sequence numbers, sliding windows, ACKs, and timeouts.
Additive Increase Multiplicative Decrease. Don't overwhelm the routers.
Path vector routing. How autonomous systems talk and occasionally break the internet.
Root servers, TLDs, authoritative servers, and caching. It's always DNS.
HTTP/1.1 vs HTTP/2, REST, and the protocol we use for literally everything now.
Content Delivery Networks, BitTorrent, and scaling to millions of users.
DDoS attacks, reflection attacks, and securing BGP/DNS.