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I have 3 simple questions:-

  1. 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.

  2. Is there a usecase to have multiple subnets behind a router? I don't see any, unless anybody else can point out otherwise.

  3. 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.

  1. Is is possible to have computers belonging to multiple subnet's connected to one router?

  2. Is there a usecase to have computers belonging to multiple subnet's connected to one router?

  3. Is there a performance benefit to have computers belonging to multiple subnet's connected to one router?

noi.m
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  • Thank you for clarifying a bit, but I'm still not seeing these three questions as having different answers than your original questions. Could you also expand on what your thought process for this is, what is driving this question? (A diagram might help as well.) I think with a little more context, we can be sure we're answering your exact question. – Brett Lykins Sep 23 '13 at 16:13
  • Hi Brett, there isn't any use case which is a driving force for this question. I had just been reading and attempting to understand sub-nets hence the question. If the answers arent different then that's ok. – noi.m Sep 24 '13 at 05:02
  • Yes, this is done with VLANs. You can break up the physical interface on the router into subinterfaces and use trunks ports on the switch to do this. This is called "Router on a Stick", google and read up on that. It's very simple. Also most beginner networking courses will cover this like ccna or network+. – allwynmasc Jun 14 '17 at 10:20

4 Answers4

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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.

Brett Lykins
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Keeping these answers deliberately simple for the simple questions asked.

Is is possible to have multiple subnets behind a router?

The very purpose of a router is for reachability to multiple other networks or subnets. You're confusing routes from connected interfaces with routes that the router knows are off in the distance (i.e., not directly connected). And when speaking of subnet masks, these are meaningless without applying to a network prefix -- 192.0.2.0/16 (255.255.0.0) is a different network from 192.0.2.0/24 (255.255.255.0). The router is a traffic cop that needs to know which paths to take; this is learned from the connected interfaces it has or through either static routes or a dynamic routing protocol.

Is there a use-case to have multiple subnets behind a router?

Stating "behind a router" doesn't mean directly connected. The use-case could be three routers R1-R3 and three subnets sA-sC as described below. The routers could be at separate physical locations.

R1
|
sA
|
R2
|
sB
|
R3
|
sC

From the perspective of R1 and looking down, sA-sC are "behind" router R1. sB-sC are behind R2, and only sC is behind R3. R1 is only directly connected to sA, R2 is directly connected to sA and sB, and R3 is directly connected to sB and sC. R1 would need either a static route or a dynamic routing protocol to learn about sC and that it should send packets destined for sC to R2 (next-hop).

Is there a performance benefit of having multiple subnets behind a router?

This is more of a scalability issue than performance. Only a limited number of directly connected subnets will go to a router which handles routing for a far larger number of networks. Performance becomes an issue on a router when Mbps (bits/sec) or Mpps (packets/sec) realistic maximums are hit among other things and less about how many subnets are behind it unless it's handling a large number of networks that requires a lot of Mbps and Mpps throughput.

generalnetworkerror
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having multiple subnets behind one router could be accomplished using a router-on-a-stick setup.

basically you configure a trunk between a router and a switch and on the router's port of the trunk you create sub-interfaces where you specify the sub-net of the specific sub-interface and what vlan should the packets be tagged as.

so now you have one link to the router, which is split into sub-interfaces that are logically separated from each-other and the router can route between them and as far as the switch on the other side is concerned it's just another trunk, except this one happens to be the one where all the gateways are.

evilgoat
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Here is a good reason to have multiple routers and subnets. We have a huge (600,000 sq feet) warehouse that is filled with access points. They all ended up going to the main router. Handheld scanners are used by pickers and seamlessly switches from one access point to another as the picker moves about the warehouse. However, they also have some regular PC workstations for printing pick tickets, returns, looking things up, etc. They don't move. For some reason they will sometimes connect to an access point 300 yards away and get a weak connection, which drops in the middle of doing work. The solution was to put a router above the workstations (all in one place) with a different subnet mask. The workstation DHCP to that subnet only (so never 300 yards away). That router then has a static route or bridge (never sure of the right term) that makes it "join" the main network. That ways the hand held scanners can get any access point they want and change them as people move, and the workstations always have a nearby router (on the ceiling, line of sight) to connect to. That router is hardwired into a switch which is hardwired back to the computer room. If you don't put the static route in they become independent which is sometimes good for security. I always heard this call a "silo" system.

It is not hard to do and gives you more control over what connects to what. We can also firewall the workstations independently from the rest of the main network -- disallowing external access in or out (they are only needed for internal access) in the case where you do connect them with a static route. If they are independent they just share the WAN. And cannot get to each other, so you have good security.

Eric
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