Stage 1 · Lesson 2 of 8

TCP, UDP and port numbers

The delivers data to the right program. TCP opens a connection with a three-way handshake (SYN, SYN-ACK, ACK), numbers every byte and resends anything lost, so it suits web pages, file transfers and SSH. UDP sends , standalone messages, with no connection and no resending, so it suits DNS lookups, voice, video and DHCP, where speed matters more than a perfect copy.

say which service a is for: 22 for SSH, 53 for DNS, 443 for HTTPS and so on. You will read ports in every access list, firewall rule and , so learn the common ones by heart.

Common trapthinking UDP is rarely used because it is "unreliable": some of the most important services on any network run over it.

A question to check yourself, an AI practice session and 2 cheat sheet entries come with the full roadmap. Unlock

Resources for this stage

Start with· follow one of these

PlatformFree
Cisco Networking Academy
Cisco · Beginner
Start with the free, self-paced Networking Basics course; it uses Packet Tracer from the first modules
VideoFree
Computer Networking: A Top-Down Approach (Video Lectures)
Jim Kurose and Keith Ross · Intermediate
Free university-level lectures from the authors of the classic textbook, for the why behind every layer

Practise

Cisco Packet Tracer
Build a two-subnet network and use simulation mode to watch ARP, DHCP and pings step by step

Keep handy

Wireshark User's Guide
Capture your own traffic and read real headers; the display filter chapter is the one to bookmark

Also worth your time

TCP vs UDP: The Key Differences Every Developer Should Know (opens in a new tab)Read
A clear comparison of the two transport protocols and when each one fits
Beej's Guide to Network ProgrammingGo deeper
If you code, write a tiny TCP and UDP client and server to see the transport layer from the inside