I can't subnet in my head, I have aggis
Although I have worked as a core network engineer for 2 national isps and know ospf/isis/bgp like the back of my hand.
I've also built an ISP from the ground up for a customer. Means getting the equipment, getting the IP from regional authority, getting fibers from local telco and negotiating the necessary last mile deals with the same entity, building the necessary servers, radius, tacacs, www, mail, dns... the works.
THAT was fun
I could also add that certs mean jack shit unless it's the best one from either Juniper or Cisco (I don't have any, my companies kept only the minimum amount of them to get the necessary discounts from cisco/jun).
Get youself some basic knowledge, like try to understand how local area network really works and how that relates to internet for example. Then get youself hired as a Junior Network Technician to a local ISP.
After that, you really start learning
PS. Well, I *could* basically subnet in my head, but it's time-consuming, error-prone and gives you an headache. It's enough that you know CIDR.