L01
Ports, protocols, clients and servers
Cram this first — memory hook
Address = building; port = service door; protocol = conversation rules.
Exam trap
A physical switch port, Transmission Control Protocol /User Datagram Protocol port, and Internet Protocol protocol number are three different things.
The slide facts
The service overview associates web browsing with 80/443, File Transfer Protocol with 21, secure file transfer with 22, and modern Server Message Block with 445. The scanning slide treats ports as service entry points.
If you have time — extra
A protocol is an agreed set of communication rules. Internet Protocol () addressing helps deliver data to an addressable network endpoint. A transport port helps the receiving computer give that data to the right service. Imagine an apartment building: the address gets you to the building and the apartment number identifies a destination inside. The analogy stops there: a port is a software identifier, not a physical socket. Transmission Control Protocol () provides ordered, reliable byte delivery while the connection remains viable. User Datagram Protocol () sends individual datagrams without -style delivery and ordering guarantees. Applications can add their own reliability or encryption over . The client usually uses a temporary source port and contacts the server's familiar destination port. The reply reverses those roles. Defaults can be changed, so a port suggests a service rather than proving it.
Walkthrough
A browser selects a temporary source port, contacts the web server on destination 443, establishes the connection and security session, and exchanges web data. The return packet goes to the client's temporary port, not another web listener on 443.