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 understand implementing IPv6 in your networks.
As one reader review stated:
This book is not going to teach you how to program network software, and it's not going to teach you how to administer network servers. It goes into horrid detail on all the bits flying around on the network. And it does so incredibly well.
Which it does—this book removes the mysteries of what happens when your bits go out across the wires.
A great paper that describes what a genuine Virtual Private Network (VPN) is, how OpenVPN is the best VPN of all, the problems with IPSec, and how the majority of pricey commercial SSL-based VPNs are not real VPNs, and not all that secure.
If you don't understand TCP/IP, you won't understand computer networking. This book is a great reference that covers essential theory as well as hands-on administration.
This is a wonderful book for all Debian users and admins that fully explores all the riches and sophisticated power tools available to Debian users.
SNMP is the common language for network hardware and software, and the key to both monitoring and management. Once you figure out all those OIDs and MIBs and community strings and traps, the rest is easy.
Ace network admins need scripting skills, and this is the book to use to learn them.
A great, 72-page downloadable PDF that covers bash commands in detail; well-organized and with clear explanations.
My very own book for Linux system administrators and power users, designed to be a companion to Linux Networking Cookbook. It covers package management; running various servers such as mail, web, DNS, and DHCP; backup and recovery; system rescue; file and printer sharing on mixed networks; and more.
A perennial classic, continually updated and containing all substantial user, programming, administration, and networking commands for the most common Linux distributions.
If you really want to be the reigning TCP/IP Master of the Universe, these books are for you. Most admins wear out several copies of Volume 1. Clear, thorough, abundantly illustrated, and a pleasure to read.
The official book of the Samba team, also available free online at http://www.samba.org/. It's especially valuable for understanding the weird stuff in Windows networking (which is pretty much all of it), and what you need to know to run Linux networks that have Windows hosts. Even if you can't migrate away from Windows desktops to nice solid Linux PCs, Windows clients on a Linux network makes all kinds of sense, and Samba is the key to making it all work.
This book goes into useful detail on using different desktop environments, running Fedora on laptops, running servers, package management, RAID, SELinux, Xen, security, and data storage.
This is targeted more at programmers than network administrators, but it's great at removing a lot of mystery from OpenSSL protocols and key management.