I'd recommend Kurose's Computer Networking: A Top Down Approach (now in 8th edition) book if you want to learn a proper computer networks concepts because this is the definitive textbook.
If you want the practical aspects of computer networks, you can try Niall Manfields' Practical TCP/IP designing, using and troubleshooting TCP/IP networks on Linux and Windows book. Granted it's a bit old (2nd edition is essentially the same 1st edition), but because you are after the essentials it should be more than fine. Here's the review of the book:
I read Kurose’s book in college and recommend it as the perfect book for this topic. As a neophyte at the time, I remember really drilling into the details of TCP and just being in awe of how well thought out the whole system was. This book laid such a good foundation for my career as a software engineer.
I really didn't like it (whatever version was current in 2004?ish), so I'd suggest Tanenbaum's Computer Networks. Not sure if it got updated in the last 15 years, but the basics should still be there.
If you want the practical aspects of computer networks, you can try Niall Manfields' Practical TCP/IP designing, using and troubleshooting TCP/IP networks on Linux and Windows book. Granted it's a bit old (2nd edition is essentially the same 1st edition), but because you are after the essentials it should be more than fine. Here's the review of the book:
https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=122...