Virtual Lans

Virtual LANs provide support for workgroups. A LAN consists of one or more LAN segments, and hosts on the same LAN segment can communicate directly through Layer 2 (link layer) without a router between them. These hosts share the same Layer 3 (network layer) subnet address, and communication between the hosts of one LAN segment remains in this segment. Thus Layer 3 (network layer) subnet address forms a broadcast scope that contains all hosts on the LAN segment.

The workgroups are groups of hosts sharing the same servers and other resources over the network. The hosts of a workgroup are attached to the same LAN segment, and broadcasting can be used for server detection, name resolution, and name reservation.

In a traditional LAN the broadcast scope is limited to one LAN segment. Switched LANs use a switch infrastructure to connect several LAN segments over high-speed backbones. Switched LANs share the Layer 3 (network layer) subnet address, but offer an increased performance compared to traditional LANs, since not all hosts of a switched LAN have to share the bandwidth of the same LAN segment. LAN segments connected over backbones allow for distribution of hosts over larger areas than that covered by a single LAN segment.

Traditional switched LANs require a separate switch infrastructure for each workgroup in the environment with several different workgroups using different LAN segments. Virtual LANs are switched LANs using software configurable switch infrastructure. This allows for creating several different broadcast scopes over the same switch infrastructure and for easily changing the workgroup membership of individual LAN segments.

The disadvantage of virtual LANs is that a switch infrastructure is needed and administration includes Layers 2 and 3 (link and network). A desirable solution involves only Layer 3 (network) and does not require special hardware.

Kurz et al. propose a flexible broadcast scope for workgroups based on Layer 3 (network). This solution uses multicast addressing, mobility support, and the DHCP for the IP. The hosts in the network are connected to routers via point-to-point connections. The features used are included in the IPv6 protocol stacks. Security can be achieved by using authentication and encryption mechanisms for the IP. Flexible broadcast can be achieved through enhancements to the IPv6 protocol stack and a DHCP extension for workgroups.

In IPv6, a special address range is reserved for multicast addresses for each scope, and a multicast is received only by those hosts in this scope that are configured to listen to this specific multicast address. To address all hosts in a certain scope with a multicast, the multicast must be made to the predefined all-nodes address, to which all hosts must listen. When existing software using IPv4 (Internet Protocol version 4) is migrated to IPv6, the IPv4 broadcasts are changed to multicasts to the all-nodes address, as this is the simplest way to maintain the complete functionality of the software.

IPv6 multicasting can be used to form the broadcast scope of a workgroup. The workgroup is the multicast group, whose hosts listen to the same multicast address, the workgroup address. A host can listen to several multicast addresses at the same time and can be a member of several workgroups.

Multicasting exists optionally for IPv4 and is limited by a maximum of hops. The multicast in IPv6 is limited by its scope, which is the address range.

In a virtual LAN, the workgroup membership of a host is determined by configuration of the switches. Kurz et al. propose that a host has to determine its workgroups and their corresponding multicast addresses. Different workgroups are separated in Layer 3 (network) since each host has the possibility to address a specified subset of hosts of the network using multicasting. All hosts can be connected directly to the routers, and the members of different workgroups can share the same LAN segment.

The administration of the workgroups is designed by storing the information about hosts and their workgroups in a central database in a DHCP server. The information is distributed by using the Dynamic Host Configuration Protocol version 6 (DHCPv6).

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