You can use java.awt.Graphics or java.awt.Graphics2D classes to draw on Java Components like, Panel or Canvas. According to your requirement you can draw Lines and Shapes to make your network diagram.
Take a peek at Graphviz and Dot to see how complex it can be to produce a neat looking graph. I think there are some comparable commercial products for Java, don't know about open source. You could even shell out to Graphviz if you liked it.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi