Theres no reference to a network in any of the lore. They do refer to Defense Forces..........so i doubt they refer to their military as a Defense Network.
...cause the lore is just about ships. just sayin.
But I like Kitkun's comment. How do you know TDN refers to just one part of the TEC forces? We don't know, and judging by the information we do have, it makes the most logical sense that it refers to all branches (if there is more than one, this is space, who knows what else besides fleets will be needed?)
And the Defense Forces thing just solidifies the "D" as being Defense. It doesn't remove any other possibilities for other letters. Forces can't go into the "N" slot, so network is still open...