If you are searching for a direct download link to version 16.1, you will encounter three major issues:
Some abandonware archivists preserve OPNET Modeler 16.1 for historical and educational purposes. The Internet Archive (archive.org) occasionally has disk images. However, these rarely include working licenses. download opnet modeler 16.1
Despite the challenges, there are legitimate pathways. Do not trust torrent sites or random file repositories claiming to offer a direct download. Instead, try the following: If you are searching for a direct download
lmadmin (license manager) if using floating licenses.modeler.exe.