Does a network need a router to pass information?

[benjamin.lawson]

So I've been arguing with a coworker whether a router is needed to pass information in a network. According to the OSI model, in order for messages to move from one place to another, it requires network layer which a switch (not including a layer 3 switch) is not capable of networking. Is there any information that can be shared between PCs with only a switch?

[Reza Sharifi]

Yes, say if you have a flat network of servers and hosts in the same vlan/subnet connected to one or multiple layer-2 switches, they can all communicate with each other without any need for a router or a layer-3 switch. If you have multiple subnets/vlan than you need a router to route between the subnets.

HTH

[benjamin.lawson]

Thank you for your quick response! How would applications work between each other since there is no layer 3 devices? Doesn't applications need layer 3 to communicate? The ping command uses ICMP protocol on layer 3. Since it is limited to layer 2 devices / protocols, what kind of information can be shared?

[johnlloyd_13]

hi,

you only need a router if you want to route packets to another IP network/subnet.

you can still communicate with another PC if they're on the same VLAN on a Layer 2 switch or to another trunked L2 switch that has the same VLAN.

[Joseph W. Doherty]

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Posting

How it works, on the end hosts, is how it would work if there were routers but they were being used in proxy mode.

[pwwiddicombe]

PING operates at Layer 3, if it has to. 

On your flat L2 only network, your devices don't need a gateway (as there is nowhere else to go).  Devices on the network will look at the destination IP and consult their own IP address and mask, and IF the destination is on the same subnet, will simply broadcast an ARP message, which will reach the destination, which will reply with a directed message with it's MAC address.  The 2 stations then happily communicate in that manner.

If you attempt a destination that is NOT in the same subnet (based on your IP and mask), then you would normally discover your gateway via the same ARP mechanism - which you don't have in a L2 network.