I don't have any network certificates. And IMO, I'm not entirely enthused about them, but I recognize they're a required checkbox for getting one's foot in the door, kinda like having a college degree, esp for certain government employers. But I digress.

My networking training was on-the-job, where my mentor basically gave me a hard-copy version of this book: The All-New Switch Book, 2nd Edition, by Seifert and Edwards. In this case, "all-new" refers to 2008. But that's alright because the fundamentals of modern computer networks have not substantially changed, even as we push beyond 400 Gbps and use MPLS to forward Metro Ethernet, or whatever.

In the end, a fundamental understanding involves switching and routing, the whole OSI layer model and practical realizations of it, Ethernet in detail, IP (Legacy + v6) in detail, and best-practices for network design. What a CCNA certificate might specifically cover is the Cisco-specific CLI syntax for setting up and maintaining a network, but knowing the fundamentals means it's easy to manage any vendor's equipment, or even virtual networks for VMs or hyperscalar cloud environments.