I have 3 simple questions:-
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Is is possible to have multiple subnets behind a router? I believe it is, e.g. router LAN interface would have subnet mask 255.255.0.0 and machines connecting to the router would have subnet mask 255.255.255.0.
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Is there a usecase to have multiple subnets behind a router? I don't see any, unless anybody else can point out otherwise.
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Is there a performance benefit of having multiple subnets behind a router? Again, i don't see any. One of the uses of a subnet is to separate traffic between different set of users. However, in this case all the traffic meant for multiple subnets is ending up on the same router.
[ADDED]
I think there some confusion due to the wording i used. Thanks for all the answers but let me rephrase the questions.
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Is is possible to have computers belonging to multiple subnet's connected to one router?
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Is there a usecase to have computers belonging to multiple subnet's connected to one router?
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Is there a performance benefit to have computers belonging to multiple subnet's connected to one router?
Best Answer
1) It is possible to have multiple subnets "behind" one router; the Internet would be a very broken place with millions more routers needed if it were not.
However, your example of using a /16 subnet mask on the LAN interface, with many /24's connecting to it, is not exactly how it would actually function. (More on that in a moment.)
2) There are many use cases. This aggregation and routing of traffic for many networks/ subnets is the purpose of a router.
Take for a simple example, a branch office location with VoIP telephones. At this site, there is a router connected via Dot1q trunking to a layer 2 access switch.
There is, of course, a need for separation of the Voice and Data traffic, which can be achieved by using separate VLANS on the access switches. Each VLAN will have a separate subnet back on our router. Each subnet would terminate on its own "sub interface", a logical partitioning of the physical connection to the access switches.
In this example, there are two subnets (one for data, and one for voice) living "behind" the router.
3) The performance benefit is that you don't need a different physical device to handle the routing for every single subnet in your network. You talk about the traffic ending up on the same router, and it does, but the traffic is still separated unless you specifically allow it to move between the subnets.