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A virtual LAN (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer (OSI layer 2).〔IEEE 802.1Q-2011, ''1. Overview''〕〔IEEE 802.1Q-2011, ''1.4 VLAN aims and benefits''〕 ''LAN'' is an abbreviation of ''local area network''. To subdivide a network into virtual LANs, one configures a network switch or router. Simpler network devices can only partition per physical port (if at all), in which case each VLAN is connected with a dedicated network cable (and VLAN connectivity is limited by the number of hardware ports available). More sophisticated devices can mark packets through ''tagging'', so that a single interconnect (''trunk'') may be used to transport data for multiple VLANs. Since VLANs share bandwidth, a VLAN trunk might use link aggregation and/or quality of service prioritization to route data efficiently. VLANs allow network administrators to group hosts together even if the hosts are not on the same network switch. This can greatly simplify network design and deployment, because VLAN membership can be configured through software. Without VLANs, grouping hosts according to their resource needs necessitates the labour of relocating nodes or rewiring data links. ==Uses== Network architects set up VLANs to provide the network segmentation services traditionally provided only by routers in LAN configurations. VLANs address issues such as scalability, security, and network management. Routers in VLAN topologies filter broadcast traffic, enhance network security, perform address summarization, and mitigate network congestion. Switches may not bridge network traffic between VLANs, as doing so would violate the integrity of the VLAN broadcast domain. VLANs can also help create multiple layer 3 networks on a single physical infrastructure. For example, if a DHCP server is plugged into a switch it will serve any host on that switch that is configured for DHCP. By using VLANs, the network can be easily split up so some hosts will not use that DHCP server and will obtain link-local addresses, or obtain an address from a different DHCP server. VLANs are data link layer (OSI layer 2) constructs, analogous to IP subnets, which are network layer (OSI layer 3) constructs. In an environment employing VLANs, a one-to-one relationship often exists between VLANs and IP subnets, although it is possible to have multiple subnets on one VLAN. By using VLANs, one can control traffic patterns and react quickly to relocations. VLANs provide the flexibility to adapt to changes in network requirements and allow for simplified administration.〔 VLANs can be used to partition a local network into several distinctive segments,〔(【引用サイトリンク】url=http://www.discovery.org.in/discoveryengineering/current_issue/v2/n8/A11.pdf )〕 for example: * Production * Voice over IP * Network management * Storage area network (SAN) * Guest network * Demilitarized zone (DMZ) * Client separation (ISP, in a large facility, or in a datacenter) A common infrastructure shared across VLAN trunks can provide a very high level of security with great flexibility for a comparatively low cost. Quality of service schemes can optimize traffic on trunk links for real-time (e.g. VoIP) or low-latency requirements (e.g. SAN). In cloud computing VLANs, IP addresses, and MAC addresses on them are resources which end users can manage. Placing cloud-based virtual machines on VLANs may be preferable to placing them directly on the Internet to avoid security issues.〔Amies A, Wu C F, Wang G C, Criveti M (2012). (Networking on the cloud ) ''IBM developerWorks'', June 21.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Virtual LAN」の詳細全文を読む スポンサード リンク
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