4

I know that the layer 2 doesn't understand IP, and the broadcast domain is same.

Why can't devices talk, how does it happen?

6
  • Are you referring to two subnets on the same VLAN, from your previous question?
    – Ron Trunk
    May 7 '17 at 19:41
  • yes in the same default VLAN
    – Nina
    May 7 '17 at 19:44
  • Sorry for the hairsplitting, but your question asks for, why devices in different subnets can't talk. The answer is: yes, they can talk. Now I am sending this comment from a very different subnet as the subnet of the stackexchange frontend serverfarm. My device, and the device of the SE frontend gateway, can communicate without any problem.
    – peterh
    May 8 '17 at 0:46
  • Furthermore, "and the broadcast domain is same.". Same as what? There is not even "broadcast domain" in l2. There is a broadcast address, which is ff:ff:ff:ff:ff:ff on Ethernet.
    – peterh
    May 8 '17 at 0:48
  • Peter, surely you they can talk usin routers but I.mean with out router.
    – Nina
    May 8 '17 at 0:49
11

Devices in different subnets can communicate. That is the purpose of a router. Routers route packets between different networks.

Even if devices in different networks are on the same layer-2 broadcast domain, you need a router to let the devices communicate at layer-3. That is because each host will compare the destination layer-3 address and its own layer-3 address and mask to see if they are on the same network. If the destination host is on a different network, the host will send the packets in layer-2 frames to its configured gateway (router).

The host must assume that the destination network could be across the world from its network, and the gateway is the host on the network that knows how to forward packets toward the destination network.

2
  • Thanks, this is Absolutely clear , I have a simple question ,if there are many networks on the switch they are considered in the same domain and broadcasts will flow to all networks ?
    – Nina
    May 7 '17 at 19:53
  • 3
    That depends. A VLAN is a broadcast domain. If you have multiple VLANs on one switch, then you have multiple broadcast domains. If you have multiple networks on the same VLAN, then layer-2 broadcasts will reach every host on the VLAN, regardless of the network. Hosts on a different network will ignore layer-3 broadcasts on a different network, but the broadcast will still interrupt every host. You use VLANs to break up the broadcast domains.
    – Ron Maupin
    May 7 '17 at 19:57
5

For Communication between different subnets packet has to reach router or layer3 devices to process this packet . If another networks is directly connected then forwards packets on basis of ARP table . If destination is on another networks means forwards packet to next hops on basis of route entry Configured on network .

However layer2 device cannot understand IP address . Only same subnets traffic can be handle by layer2 device, traffic is forwarded based on mac-address table

-2

In linux...

Three computers, one has two interfaces, the other two on different subnets. The one with two is technically a router in function. And a router would be simpler.

The missing parts for me was the -A FORWARD in both directions on the bridging computer for iptables. And the remote clients need a route to respond to each other.

ip route add 192.168.2.0/24 via 192.168.7.1

for one client

ip route add 192.168.7.0/24 via 192.168.2.1

for the other client

Where the bridging computer has two interfaces, one statically assigned 192.168.2.1 and one statically assigned 192.168.7.1.

I needed this to use my pulseaudio server that I put on a different subnet and because I ran out of ports on various switches / routers. And I wanted to keep audio/file traffic different from internet traffic.

2
  • This doesn't answer the question why.
    – Zac67
    May 25 '20 at 18:04
  • 1
    "The one with two is technically a router in function." is wrong. A device with 2 interfaces in different subnets is not necessarily a router. It is a multi-homed host. The fact that linux has routing capabilities is a different matter.
    – JFL
    May 25 '20 at 18:17

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