In what scenarios would a VLAN not align 1:1 with a subnet?

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?

Solution 1:

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.