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Anetwork access server (NAS) is a group of components that provides remote users with a point of access to a network.^[1][2]

A NAS concentrates dial-in and dial-out user communications. An access server may have a mixture of analog and digital interfaces and support hundreds of simultaneous users. A NAS consists of acommunications processor that connects asynchronous devices to aLAN orWAN through network andterminal emulation software. It performs both synchronous and asynchronous routing of supported protocols.

The NAS is meant to act as agateway to guard access to a protected resource. This can be anything from atelephonenetwork, toprinters, to theInternet. Aclient connects to the NAS. The NAS then connects to another resource asking whether the client's suppliedcredentials are valid. Based on that answer the NAS then allows or disallows access to the protected resource.

The above translates into different implementations for different uses. Here are some examples.

• AnInternet service provider which provides network access via common modem or modem-like devices (be itPSTN,DSL,cable orGPRS/UMTS) can have one or more NAS (network access server) devices which acceptPPP,PPPoE orPPTP connections, checking credentials and recording accounting data via back-endRADIUS servers, and allowing users access through that connection.
• Thecaptive portal mechanism used by manyWiFi providers: a user wants to access the Internet and opens abrowser. The NAS detects that the user is not currentlyauthorized to have access to the Internet, so the NAS prompts the user for their username and password. The user supplies them and sends them back to the NAS. The NAS then uses theRADIUS protocol to connect to anAAA server and passes off theusername andpassword. The RADIUS server searches through its resources and finds that the credentials are valid and notifies the NAS that it should grant the access. The NAS then grants the user access to the Internet.
• Another use of a NAS would be invoice over IP (VoIP). However, instead of using a username and password, many times aphone number orIP Address are used. If the phone number is a valid customer then the call can be completed. Other uses might be to verify whether a phone number haslong distance access or atelephone card has minutes left.

Although not required, NASs are almost exclusively used withauthentication, authorization, and accounting (AAA) servers. Of the AAA protocols available,RADIUS tends to be the most widely used. TheDiameter base protocol extends RADIUS services by providing error handling and inter-domain communications. This protocol is used in networks like theIP Multimedia Subsystem (IMS).

• Terminal server

• RFC 2881, Network Access Server Requirements Next Generation (NASREQNG)

• Network access
• Telephony
• Telecommunications infrastructure

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