NEF files can be opened using Nikon ViewNX2. You have two basic choices: either update your program to a new version, or use a converter to change the format.
NEF file using a program made before the D7100 was released, it will not be able to understand the format. A new camera means a new sensor and new data and settings. The reason for this is that a RAW (.NEF) file contains the output taken directly from the camera sensor as well as the camera specific settings you use to take the shot.
With each new camera, the RAW data changes, and software must be updated accordingly to be able to read the file. Photos taken in RAW mode are stored in files with the. The Nikon RAW image format is Nikon Electronic Format ( NEF).