whats a vlan?

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PK-her0

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ok so wiki gives this discription...

VLANs are created to provide the segmentation services traditionally provided by routers in LAN configurations. VLANs address issues such as scalability, security, and network management. Routers in VLAN topologies provide broadcast filtering, security, address summarization, and traffic flow management. By definition, switches may not bridge IP traffic between VLANs as it would violate the integrity of the VLAN broadcast domain.


This is also useful if one wants to create multiple Layer 3 networks on the same Layer 2 switch. For example if a DHCP server (which will broadcast its presence) were plugged into a switch it would serve anyone on that switch that was configured to do so. By using VLANs you easily split the network up so some hosts won't use that server and default to Link-local addresses.

Virtual LANs are essentially Layer 2 constructs, compared with IP subnets which are 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 or have one subnet spread across multiple VLANs. Virtual LANs and IP subnets provide independent Layer 2 and Layer 3 constructs that map to one another and this correspondence is useful during the network design process.

By using VLAN, 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.

and thats all very nice...

1) But how are they implemented in a real world scenario?
2) Why would you implement one?
3) What do you need to implement one?

some general info please...i hate technical terminology...(though i should get used to it)
 

zx10guy

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VLANs or virtual LANs are logical groupings of layer 2 networks which can all co-exist on the same physical layer 2 network. VLANs are used when you want network traffic isolation from one group of devices over other devices but don't want to deal with the complexities or the expense of having dedicated physical hardware for each isolated layer 2 network you want to create. VLANs also aid in network performance and tuning by creating individual layer 2 broadcast domains. Instead of exposing your entire layer 2 infrastructure to a multicast/broadcast which should only be directed to a section of your network, a VLAN contains this traffic to ports/devices designated on that VLAN.

VLANs also allow you to easily extend and scale your network. Say you have a group of people in the accounting office which need to be set up in a different area in your office building. Instead of needing to home run dedicated drops to the specific switch dedicated for that group, you would just extend the VLAN to that switch and then assign the ports the accounting people are using to that VLAN.

VLANs are now standardized to follow IEEE 802.1Q. Before 802.1Q was set in stone, Cisco came out with their own interim proprietary solution called ISL. ISL can only function between Cisco devices with ISL support, but ISL is going the way of the Do Do. VLANs can only function on switches which understand the 802.1Q formated frames. You cannot pass VLAN traffic through switches with no 802.1Q (or VLAN tagging) support. Switches that do support 802.1Q are managed switches. To have traffic from one VLAN move into another, you must have a layer 3 device which routes traffic between the different VLANs.
 

PK-her0

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thanks again john,

so if you have a lan and a router connecting to another router and a lan,this isnt a vlan?
what is this called...when i connect to another network via a router?

thanks again...
 

JohnWill

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The closest term I can come up with is perhaps gateway. I never tried to name it. :D
 

zx10guy

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thanks again john,

so if you have a lan and a router connecting to another router and a lan,this isnt a vlan?
what is this called...when i connect to another network via a router?

thanks again...
No. In your described situation, it can be either a WAN setup or a intranet.
 

zx10guy

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One other thing to understand. VLANs are formed by marking frames as they are introduced into the layer 2 environment. The frames are modified at the header to contain an ID stating which VLAN the frame belongs to. This is how an 802.1Q complaint switch understands how to handle and direct the traffic tagged accordingly...hence the term used with 802.1Q which is VLAN tagging. This alteration of the header of a frame is why regular switches cannot process VLAN tagged frames.
 
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