You can use Dia to draw network diagrams in linux or windows. However, if you just want to use LibreOffice Draw or Impress or even Open Office, you can use network images from Cisco.
You can them at http://www.cisco.com/web/about/ac50/ac47/2.html
You will also find Visio templates at http://www.cisco.com/en/US/products/hw/prod_cat_visios.html
You can see the diagrams created using the them in my post on How to extend range of Wireless network using dd-wrt and another router for free.