As a primer on networking in general, I'd recommend TCP/IP Network Administration, Third Edition, by Craig Hunt. This book provides a chapter on the TCP/IP stack, another on Addressing and routing and the remainder of the book covers in reasonable depth most common network services and diagnostic tools.
For a heavyweight reference, get TCP/IP Illustrated, Vol 1: The Protocols, by Richard Stephens, if you become obsessed with networks buy or borrow volumes 2 and 3.
As far as Java specific networking introduction, I'd suggest Java Network Programming, Third Edition, by Elliotte Rusty Harold, this book does take some critiscim but I still believe it's a good introduction and is an approachable read.
