(Non-Secure Internet Protocol Router NETwork) The U.S. DOD network for exchanging "sensitive but unclassified" information. A huge private network, NIPRNet evolved from MILnet, which was split off from ARPAnet in the early 1980s. Nipper and Sipper The classified counterpart of NIPRNet is SIPRNet (the "S" for Secret). The two networks are known as "NIPPERnet" and "SIPPERnet" or "Nipper" and "Sipper." See ARPAnet.