Note:
I've used these shapes only with Dia-win32-v0.88 and really don't know if they'll work with other versions or platforms (I assume they would but haven't tested it).

network2.tar.gz

Vague directions:
  1. Save above file to dia directory
  2. unzip & untar (once completed there should be a new sheet file in the "sheets" directory and a new shape directory under "shapes")
  3. Restart dia



Here's a look at the shapes themselves (images in .png format):
generic_router:  
generic_switch:  
generic_switchengine: