Wireshark User's Guide
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Official free resource for protocol dissection
More resources on Network Protocols
Cloudflare Learning: Network Layer
Free articles on network protocols like IP
Beej's Guide to Network Programming
Free guide to socket programming and protocols
How the Internet Works in 5 Minutes
Simple explanation of network protocols
Introduction to TCP/IP
You use the Internet through your PC (Personal Computer), laptop, tablet, smartpad, and smartphone every day in everything you do. Through your own PC/laptop, you can easily learn everything about the Internet, and that is what this course is focused on. In this course ‘Introduction to TCP/IP,’ you will learn the operational functions of Internet technologies (which include IPv4, IPv6, TCP, UDP, addressing, routing, domain names, etc.) and your PC/laptop's security and gateway Internet setup and basic principles. In addition, through a simple Wireshark experiment, you will see the TCP/IP packets and security systems in action that are serving your PC/laptop, that serves you.
geeksforgeeks.org
GeeksforGeeks is a computer science education site offering tutorials, articles, and practice problems across programming, algorithms, data structures, and networking. It covers networking fundamentals and network protocols with step-by-step explanations of topics like the OSI model and TCP/IP, plus hands-on examples and quizzes to reinforce learning.
Network Protocols
Coursera course: Networking Fundamentals and Protocols