Networks, layers, addresses, names and ports

Use models to organize responsibilities

The OSI model has seven conceptual layers: physical, data link, network, transport, session, presentation and application. The Internet model in the selected IETF architecture has link, internet, transport and application layers. These are ways to reason about functions, not a promise that each real product implements seven separate pieces. Encapsulation adds the relevant protocol information as data passes through a stack.

Keep delivery and addressing distinct

IP supports addressing and packet delivery between networks. IPv4 addresses have 32 bits; IPv6 addresses have 128. Modern prefixes must be understood in their actual network context rather than by assuming old classful defaults. TCP provides an ordered, reliable byte stream; UDP does not itself provide TCP’s delivery guarantees. A network packet is not always the same size, and IPv6 does not automatically encrypt or secure all traffic.

Names and ports are different identifiers

DNS uses domain names, resource records, authoritative servers and resolvers. A name does not necessarily correspond to one website or one IP address. A transport port identifies an endpoint within the relevant protocol context; a familiar port number does not prove that the expected application is running or that its traffic is safe.

The seven OSI responsibilities

From top to bottom: application offers services used by applications; presentation concerns data representation; session concerns managing dialogues; transport concerns endpoints and processes; network concerns addressing and routing between networks; data link concerns local delivery and framing; physical concerns signals and the medium. These are conceptual responsibilities, and implementations may combine functions.

Connect the terminology.