Not only networking unless you just wanna be a network admin. But networking is one of the most important components, you need the basics to be a good sys admin regardless.

Understand IP addressing, subnets, DHCP, DNS, OSI model, basics of packet anatomy, basics of routing and switching.

Have a solid understanding of those things plus a few other networking subjects, you'll be able to troubleshoot really well. Don't be the sys admin who barely understands IP addressing, I've known that person...not good.