Ports, TCP and UDP: how two programs talk
An address names a machine, a port names a program — and a connection is nothing more than two kernels agreeing.
"Connection" is the word that misleads: nothing is stretched between the two machines, and no device along the path knows you are connected. This course shows what picks out one program among thousands, what TCP actually does before a single byte of yours moves, why UDP does without all of it quite happily, and how a machine obtains an address before it has one to be addressed by.
What you will be able to do
- Read the four values that identify a connection, and say why two clients on the same port never get confused
- Describe the three-way handshake, and what each of its three messages proves
- Choose between TCP and UDP for a given service, and justify it with something better than "TCP is reliable"
- Explain how a machine obtains an address, a prefix, a gateway and resolvers having configured none of them
- Name the five steps between a typed name and a transmitted byte, and the course that owns each one
Before this course
Taken from the learning path this course belongs to: the course placed just before it. A suggested order, not a lock — nothing here closes until you have finished it.
Outline
- 01
Naming one program among many
The port, the four values that identify a connection, the handshake, and the choice between TCP and UDP.
- 02
Getting configured, and getting connected
How a machine obtains its settings having none, and the five steps between a name and a byte.
Technical content last reviewed on 20 September 2026