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{{redirect|NAS server|the storage device|Network-attached storage}}
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A '''network access server''' ('''NAS''') is a group of components that provides remote users with a point of access to a network.<ref>{{Cite book |last=Clark |first=Martin P. |url=https://books.google.com/books?id=HccWEdLdacwC&pg=PA542 |title=Data Networks, IP and the Internet: Protocols, Design and Operation |date=2003-05-07 |publisher=[[John Wiley & Sons]] |isbn=978-0-470-84856-2 |language=en |page=542}}</ref><ref>{{Cite book |last=Held |first=Gilbert |url=https://books.google.com/books?id=06uBL8vGpoIC&pg=PA205 |title=Network Design: Principles and Applications |date=2000-05-31 |publisher=[[CRC Press]] |isbn=978-1-4200-9375-9 |language=en |page=205}}</ref>

==Overview==
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 a [[communications processor]] that connects asynchronous devices to a [[LAN]] or [[Wide area network|WAN]] through network and [[terminal emulation]] software. It performs both synchronous and asynchronous routing of supported protocols.

The NAS is meant to act as a [[Gateway (telecommunications)|gateway]] to guard access to a protected resource. This can be anything from a [[telephone]] [[Telecommunications network|network]], to [[computer printer|printers]], to the [[Internet]]. A [[Client (computing)|client]] connects to the NAS. The NAS then connects to another resource asking whether the client's supplied [[credentials]] are valid. Based on that answer the NAS then allows or disallows access to the protected resource.

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

*An [[Internet service provider]] which provides network access via common modem or modem-like devices (be it [[PSTN]], [[DSL]], [[cable modem|cable]] or [[GPRS]]/[[UMTS]]) can have one or more NAS (network access server) devices which accept [[Point-to-Point Protocol|PPP]], [[PPPoE]] or [[PPTP]] connections, checking credentials and recording accounting data via back-end [[RADIUS]] servers, and allowing users access through that connection.
* The [[captive portal]] mechanism used by many [[WiFi]] providers: a user wants to access the Internet and opens a [[web browser|browser]]. The NAS detects that the user is not currently [[authorized]] 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 the [[RADIUS]] protocol to connect to an [[AAA protocol|AAA]] server and passes off the [[username]] and [[password]]. 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 in [[voice over IP]] (VoIP). However, instead of using a username and password, many times a [[phone number]] or [[IP 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 has [[long distance calling|long distance]] access or a [[telephone card]] has minutes left.

==Associated protocols==
Although not required, NASs are almost exclusively used with [[AAA protocol|authentication, authorization, and accounting]] (AAA) servers. Of the AAA protocols available, [[RADIUS]] tends to be the most widely used. The [[Diameter (protocol)|Diameter]] base protocol extends RADIUS services by providing error handling and inter-domain communications. This protocol is used in networks like the [[IP Multimedia Subsystem]] (IMS).

==See also==
* [[Terminal server]]

==References==
{{reflist}}

==External links==
* {{IETF RFC|2881|link=no}}, Network Access Server Requirements Next Generation (NASREQNG)

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[[Category:Network access]]
[[Category:Telephony]]
[[Category:Telecommunications infrastructure]]