mahmoodmkl Sat, 05/24/2008 - 02:15
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Hi


Vlan is broadcast domain i.e all the devices are in single broadcast domain so with a broadcast domain we a have subnet associated with it so each broadcast domains specifies a subnet.


Thanks

Mahmood

Giuseppe Larosa Sat, 05/24/2008 - 03:13
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Hello,

a VLan is a group of switch ports in one or more switches that share a broadcast domain : if a frame with a broadcast destination address (ffff.ffff.ffff) is received on one port it will be forwarded to all the other ports that are associated with the same vlan. Ports belonging to different VLans cannot comunicate directly without the routing action of an intermediate device (router or multilayer switch).

An IP subnet is a contiguous block of IP addresses identified by the network, subnet and host part of the ip address.

An IP subnet has a base address host part =000..0 and a directed broadcast where host part is 111..11 (host part bits are all set to 1).

Usually in networks we use a one-to-one corrispondence : 1 subnet -- 1 vlan.

But there are some special cases: if using secondary ip addresses multiple subnets are supported by one vlan. If using inter-vlan bridging with IRB a single IP subnet can span over multiple vlans.


hope to help

Giuseppe

Richard Burts Sat, 05/24/2008 - 10:59
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Giuseppe


One part of your response puzzles me. When you talk about IRB and inter-vlan bridging I am not sure what you are describing. IRB bridges for a single bridge-group. And a bridge-group is a broadcast domain which is a VLAN. I am not sure how you would get a single subnet over several VLANs.


foxbat


I will suggest a slightly different approach to understanding the difference between a VLAN and a subnet. A VLAN is a layer 2 entity and it operates entirely at layer 2. As previous responses have pointed out a primary characteristic of VLANs is that they are a layer 2 broadcast domain.


A subnet is a layer 3 entity and it operates entirely at layer 3. Members of a subnet are assumed to be connected together in a single network and are assumed to be able to communicate directly with each other.


In general we tend to assume that there is a correspondence between VLAN and subnet and in most implementations it is true that the membership of a VLAN is exactly the same as the membership of a subnet. But there are exceptions.


HTH


Rick

Giuseppe Larosa Sun, 05/25/2008 - 03:36
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Hello Rick,

I just wanted to provide an example of an IP subnet that extends on more then a Vlan (2).

You are right that I put together two different features IRB and inter-vlan bridging but I used them in different times not together.

But in all this I was thinking of Vlans as configuration objects on switches that is a different matter:

bridge two VLans = merge the two L2 broadacast domains


I agree with you that VLAN is layer2 concept and an IP subnet is a Layer 3 one.


Best Regards

Giuseppe

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