Multiple Subnets sharing a switch (no VLAN)

[taraz0007]

Is it possible to have multiple subnets sharing a single switch (without VLANs or routers)?

If so, what would happen?

Would the broadcast traffic be restricted to a single subnet, or would both subnets recieve the broadcast traffic (e.g., one broadcast domain or two)?

Would the subnets be completely isolated from each other?

Would the subnets be able to communicate?

Would the switch work, even though some of the devices would have a different network address?

[InayathUlla Sharieff]

Is it possible to have multiple subnets sharing a single switch (without VLANs or routers)?

Answer: NO.

2-Would the broadcast traffic be restricted to a single subnet, or would both subnets recieve the broadcast traffic (e.g., one broadcast domain or two)?

Answer: Broadcast traffic will be restricted to a single subnet. But here is the theory if users are been configured with the same subnet then without VLAN they will be able to communicate with each other.

Which means single broadcast domain.

Example:  --- Switch---

                  |              |

   PC1 192.168.0.1   PC2-192.168.0.2

These two machines will be able to communicate between each other but if you have PC is 192.x subnet and PC2 is other subnet like 10.x /172.x then the ping will fail which means there will not be any communication.

3-

Would the subnets be completely isolated from each other?

Answer":-- YES

4- Would the subnets be able to communicate?

Answer:- NO

5- 

Would the switch work, even though some of the devices would have a different network address?

Answer:- No it wont work untill you do the intervlan routing or proper communication on the switch.

HTH

Regards

Inayath.

[Madhukrishnan Gopinathan Nair]

Would the broadcast traffic be restricted to a single subnet, or would both subnets recieve the broadcast traffic (e.g., one broadcast domain or two)? 

Ans : Its the same broadcast domain since you have only one vlan (dafault vlan 1).

         The broadcast send by a pc will reach another pc eventhough it has different subnet.

Would the subnets be able to communicate?

Ans : Since you dont have multiple vlans or rouer that means you dont have a gateway ip address in you PC. So it cannot communicate to other subnets. A PC himself can understand whether the destination is on same subnet or different. If its different subnet, it looks for Gateway configured or not. In your case it will fail.

Hope this helps.

Thanks,

MAdhu

[DaveVA]

If I have some PCs with 192.168.2.xxx addresses and some with 192.168.10.xxx addresses, can they share a switch whose address is in the 192.168.2.xxx address space?  Can they communicate with each other?

[Joseph W. Doherty]

For communication purposes, whether hosts can intercommunicate, between subnets, on the same L2 domain, again, depends on how the hosts are configured, and how the communication is being attempted.  The switch, as a host (i.e. its management IP), should behave like any other host.

[Richard Burts]

There is not a clear or simple answer to the question of whether 192.168.2.0 devices can communicate with 192.168.10.0 devices. It depends on several tings. One factor is whether all of the devices are in the same layer 2 broadcast domain (in the same vlan)? If they are in the same broadcast domain then communication MIGHT be possible but if they are in different broadcast domains then communication is not possible without a layer 3 routing device.

Probably the most important factor is whether each device would arp for an IP address that was outside of its own subnet. For many versions of many OS this is not possible. The device needs a functioning default gateway to forward to a remote subnet. For some versions of some OS it is possible to arp for a destination in a remote subnet. For these devices communication might be possible. So it really depends on the OS and how the device is configured.

[Richard Burts]

I have one more observation about this question: the answer to the question does not involve anything about the switch. The answer depends entirely on the hosts involved and what their capabilities are.

[Joseph W. Doherty]

Rick, last posting, and mine, might cause a bit of confusion since @DaveVA asks about the impact the switch's management IP would have.

What both Rick and I are saying, switch's management IP matters not to whether hosts on the two networks can intercommunicate.

It does matter, whether those two networks, on that switch, are in the same L2 domain, or not.

It also matters, for the switch, as its own network host, to what other networks it can intercommunicate with.  Again, basically, the rules that apply to other hosts on the 192.168.2.0/24 network also apply to the switch, itself, as its own host.

[Richard Burts]

Joseph

Yes your response in '15 and my recent response are indeed quite similar. @DaveVA posted more recently and appears not to have benefitted from your excellent post. So I wanted to attempt a slightly different approach in responding hoping that it might be helpful. Not sure what confusion you refer to?

[Joseph W. Doherty]

"Not sure what confusion you refer to?"

Just that DaveVA was also including the switch's IP being in one of the networks being inquired about.  Just wanted to make abundantly clear that the switch, as a host, would use the same intercommunication rules that apply to the other hosts.  As you wrote (correctly) " . . .the answer to the question does not involve anything about the switch." but that doesn't pertain to the switch as its own host.

Perhaps, I'm creating more confusion, by my attempt to preclude it.(?)

[Joseph W. Doherty]

Yup.  Rick's answer is variant, I believe, of my '15 posting.  (Rich, though, raises an interesting point about some hosts' OS not supporting ARP for non-local networks when they don't have a default gateway defined.  [Something, I'm unaware of.]  BTW, if hosts do have a default gateway, but it's not responding, then communication should fail to other hosts within the same broadcast domain, but in a non-local network.)

[Reza Sharifi]

You can have multiple subnets, but you at least need one vlan.  That vlan could be vlan 1 (default) with multiple secondary interfaces. So you just know, this is not a very common design.  Usually you use one vlan per subnet.

HTH

[Joseph W. Doherty]

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Posting

Is it possible to have multiple subnets sharing a single switch (without VLANs or routers)?

Yes, although a managed switch will likely have a default VLAN.  (Unmanaged switches, don't have VLANs.)

If so, what would happen?

Hosts on the same logical subnet should be able to communicate.  Hosts on different logical subnets will depend on whether they've been configured with a gateway or not.  As you posit (I believe) no active gateway, hosts configured with an inactive gateway will be unable to communicate with hosts on another logical subnet.  Hosts configured without an active gateway, should ARP for the destination, and then be able to communicate with it.

Would the broadcast traffic be restricted to a single subnet, or would both subnets recieve the broadcast traffic (e.g., one broadcast domain or two)?

All hosts would see all broadcast traffic.

Would the subnets be completely isolated from each other?

No.

Would the subnets be able to communicate?

See answer to "If so, what would happen?", above.

Would the switch work, even though some of the devices would have a different network address?

Depends what you mean by "work" and network addresses.  Remember a switch is a L2 device, so it will work for L2 frames.

[quocbinbin@gmail.com]

yes, beacause switch working using mac-address, if will learn mac source and forward to destination mac, not care ip address

[Joseph W. Doherty]

". . . will learn mac source and forward to destination mac, not care ip address"

Hmm, not quite.  Host would already know its own source MAC, and does need to learn/discover destination MAC, which is usually done how?