This work examines the factors that need to be considered when designing a national research and education network (NREN). It discusses the needs of the research and education community and what makes a NREN unique from other networks. It considers the different options available regarding transmission mediums, - technologies and higher level network protocols, and suggests principles that should be followed to obtain an optimum design. These design principles are then applied to the specific case of designing a 100 Gbps+ NREN for South Africa. The network is currently still under construction with funding from the national government, and preliminary findings regarding the implementation of the network are given.