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A basic form of NAC is the 802.1X standard. Network access control aims to do exactly what the name implies—control access to a network with policies, including pre-admission endpoint security policy checks and post-admission controls over where users and devices can go on a network and what they can do.
A typical (non-free) WiFi connection is a form of NAC. The user must present some sort of credentials (or a credit card) before being granted access to the network. In its initial phase, the Cisco Network Admission Control (NAC) functionality enables Cisco routers to enforce access privileges when an endpoint attempts to connect to a network.
Some protocols are NAK-based, meaning that they only respond to messages if there is a problem. Examples include many reliable multicast protocols which send a NAK when the receiver detects missing packets [4] or protocols that use checksums to verify the integrity of the payload and header. Still other protocols make use of both NAKs and ACKs.
NAC—Network Access Control; NACK—Negative ACKnowledgement; NAK—Negative AcKnowledge Character; NaN—Not a Number; NAP—Network Access Protection; NAS—Network-Attached Storage; NASM—Netwide ASseMbler; NAT—Network Address Translation; NCP—NetWare Core Protocol; NCQ—Native Command Queuing; NCSA—National Center for ...
Cisco Technology Handbook: SDLC and Derivatives: SDN: Software-defined networking Architecture Software-defined networking: SFD: Start-of-frame delimiter (Ethernet, HDLC, etc.) Link layer IEEE 802.3 (Ethernet), or RFC 2687 (HDLC), for examples SFP: Small form-factor pluggable Hardware Seagate Specification: S-HTTP: Secure HTTP (rarely used)
Any system failing the checks will be denied general access to the network and will probably be placed in a quarantined role (how exactly a failed system is handled depends entirely on how the Clean Access Manager is configured, and may vary from network to network. For example: a failed system may simply be denied all network access afterward).
It thus forms the basis of most of the link layer (OSI layer 2) networking upon which upper-layer protocols rely to produce complex, functioning networks. Many network interfaces support changing their MAC address. On most Unix-like systems, the command utility ifconfig may be used to remove and add link address aliases.
Examples of physical networks are Ethernet networks and Wi-Fi networks, both of which are IEEE 802 networks and use IEEE 802 48-bit MAC addresses. A MAC layer is not required in full-duplex point-to-point communication, but address fields are included in some point-to-point protocols for compatibility reasons.