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Under what scenarios would a VLAN not align with a subnet. I've been told that genearlly they always go 1 to 1. What are some examples or scenarios, and reasons for anything besides that, and for what benefits or losses?

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A VLAN is a layer 2 construct - generally an Ethernet broadcast domain mapped onto a set of ports on one or more switches. An IP subnet is a layer 3 construct and is a collection of hosts within a common address grouping with local reachability. An IP subnet can run on a VLAN, but it can also run on a wide variety of other media.

It is possible for two or more IP subnets to run concurrently on a single VLAN as long as the subnets do not overlap. This often occurs in environments migrating from one addressing scheme to another or when port space / VLAN capacity are limited. Typically the mechanism in use is secondary IP addressing for one or more hosts - usually a router. A secondary address is equivalent to an IP alias and is essentially just a single network interface with addresses in multiple subnets.

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If the ARP requests on my upstream cable connection are any indication you can add "...or when your ISP engages in a masochistic level of oversubscription" to the second paragraph. – Gerald Combs May 25 '12 at 19:23
An excellent example of the difference between "can" and "should" – rnxrx May 25 '12 at 19:25
@GeraldCombs: Today's CMTS' terminate quite a number of physical down- and upstreams. Your ISP can have thousands of customers on the same network interface without too much oversubscription. – Oliver May 25 '12 at 19:37
The opposite is also true. For example, at my workplace we use a single class B subnet, but it is split across multiple VLANs for security reasons. The router uses proxy arp to glue the VLANs together. – Harry Johnston May 26 '12 at 23:30

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