Ask HN: Best resources to learn computer networking?

I never took a formal course on computer networking and have just learned bits and pieces along the way. However, I would like to fill in the gaps in my knowledge and learn more about networking, specifically in TCP/IP, CIDR, subnets, network masks, routers, gateways, NAT, firewalls, etc. I assume most of these topics would be covered in a networking 101 course of some kind.

Any suggestions on the best resources for studying these topics? Could be a book, an online course, a website with articles, etc.

Any suggestions are much appreciated. Thanks.