Client vs Server

Why they differ

The client asks and starts the exchange; the server waits and answers. The same machine can play either role.

Networking

The program or device that starts a network exchange by sending a request, such as a web browser asking for a page.

Formal

The party in a network exchange that opens the connection and sends requests, following a shared protocol, to a server that waits and replies. The role is defined by who starts the exchange, not by the kind of machine.

In plain English

Like a customer walking up to a shop counter - the customer asks, the shop answers, and it is always the customer who starts.

In practice

In a municipality's citizen service, the program on a clerk's PC is a client - each time she looks up a citizen, it sends a request to the central case system and shows the reply.

Why it matters

Clients run on the devices people use every day and process whatever comes back to them, so a tricked or outdated client is often how an attacker first gets inside.

Server

Networking

A computer or program that waits for requests over a network and answers them, such as sending a web page or storing email.

Formal

A program, or the machine running it, that listens on a port for incoming requests from clients and returns responses according to an agreed protocol.

In plain English

Like the counter at a library - it stays open, waits for people to ask for a book, and hands over what they asked for.

In practice

A region runs its patient record system on servers in two data centres; thousands of computers at its hospitals send requests to them all day, and if one centre fails, the other takes over.

Why it matters

Servers hold the shared data and services everyone depends on and can be reached at all times, which makes them prime targets; they usually count among an organisation's critical assets.

Shared connections

API HTTP HTTPS Network