Computer Networks

Network models split communication into layers, each offering services to the layer above and adding its own header; the seven-layer OSI model is the reference, while the four-layer TCP/IP model is what the internet actually runs.

The data-link layer delivers frames between directly connected nodes using MAC addresses, controls access to a shared medium (e.g. CSMA/CD), and detects transmission errors with checksums like CRC.

IP addressing identifies hosts and networks; a CIDR prefix splits the 32-bit IPv4 address into network and host parts, and routers forward packets by the longest prefix match against their routing tables.

The transport layer provides process-to-process delivery: TCP adds connections, reliability, ordering and flow/congestion control, while UDP is a thin, connectionless best-effort service; port numbers distinguish applications.

Application-layer protocols define how programs exchange meaningful messages — HTTP for the web, DNS for name resolution, SMTP/IMAP for email — building on the transport layer to deliver services users actually see.