The RADIUS server is usually a software application running on a operating system. RADIUS appliances with simplified maintenance and management interfaces are also available. In either case, the function of the server is identical: the server waits for a request from an NAS, processes or forwards the request, and then returns a response to the NAS. The response can contain authorization policies or an acknowledgment of accounting data received.

A single RADIUS server can receive and process many simultaneous access requests from numerous types of NASs (such as ADSL, dial-up, or VPN concentrators) in many different locations. A single server may also interact with flat files, SQL databases, LDAP directories, or other RADIUS servers. In order to make a decision regarding an access request, the RADIUS server must first use information from many sources.

Once the server makes a decision, it returns a response to the NAS. The NAS may enforce the policy in that response, or it may ignore it altogether. The server has no way of knowing if the NAS has received its response, or if the NAS is obeying the instructions in that response. Since it is customary for the NAS to log very little information about what has been received or how server responses are processed, it is very difficult to create and debug local site policies.

The RADIUS server has no control over what the NAS does with a response. The NAS may follow the instructions in the response, or disregard the response, and do something different.

Consider the following analogy to help illustrate the point: a Human Resources (HR) department acts like a RADIUS server, by setting policies, and a security guard acts like the NAS in a network, by carrying out those HR department policies.

In this example, the company policy is that when an employee is fired, HR notifies security and removes building access from that employee. The security guard is then responsible for ensuring the fired employee no longer accesses the company building. If one day an employee gets fired (similar to a user being denied access) and the HR department informs the security guard that the employee is no longer free to come and go (similar to the RADIUS server decision sent to the NAS), it is then up to the security guard at the company front desk to perform the task of refusing entry to the fired employee (similar to the NAS enforcing system access in a network).

In the network, the NAS enforces system access. The RADIUS server does little more than offer advice to the NAS. The NAS may choose to ignore the response. In that case, there is very little than can be done other than to fix the NAS so that it follows instructions correctly. No amount of changes to the RADIUS server will make the NAS behave properly.