Computer networking is a deep and complex subject. Please refer to the references here for deeper study of the subjects covered in this book.
Always start here to learn iptables. You’ll see a lot of crazy iptables tutorials out there-stick with the master. This goes hand-in-hand with having an in-depth understanding of TCP/IP.
SSH is very flexible and capable, and it seems there is nothing you can’t do with it. This books does a great job of covering all of it.
This takes a more practical approach, and covers essential services like mail services, web services, name services, PPtP, and iptables firewalls.
Spend the money. Buy the book. LDAP is complex and abstract, and you’ll make yourself crazy trying to learn via the usual cheapskate channels online. You won’t be sorry.
IPv6 is coming whether you want it to or not. Ms. Hagen has done a great job of teaching IPv6 fundamentals clearly and understandably. This is the background you’ll need to ...