This is an open standard protocol that works in a client and server model. In the implementation of Cisco, the RADIUS client is configured on the Cisco routers and sends authentication or authorization requests to a RADIUS server which is located centrally. 

RADIUS can be implemented in various network environments that are in need of high security levels. Some of the environments where RADIUS can be used are as follows:

1. It can be implemented in networks that are built with different vendor products. RADIUS can act as a single server-based database.
2. In networks environments where smart cards are used.
3. It can be used in environments where administrators need to do accounting independently.
4. It can be used in networks where administrators want to set up pre-authentication profiles. Pre-authentication mainly helps ISP's to manage ports and shared resources depending on the agreed upon service agreements.

On the other hand, RADIUS cannot be used for some situations, and they are as follows:

1. RADIUS does not support some of the protocols like AppleTalk Remote Access (ARA), X.25 PAD connections, and NetBIOS
2. RADIUS does not work on the two-way authentication model
3. RADIUS binds the user client to only one service model and does not support a variety of services