Networking core concepts

L1 – Physical: Cables, NICs, bits. L2 – Data Link: MAC addresses, switches, VLANs, STP. L3 – Network: IP, routing, subnets, CIDR, ARP. L4 – Transport: TCP/UDP, ports, handshake, retransmission. L5 – Session: Connection management. Rare in practical ops. L6 – Presentation: Encryption, compression, TLS framing. L7 – Application: HTTP, DNS, SMTP, gRPC.