Why is router required to communicate between two subnets ?

[Ravi VK]

Hi ,

I am trying to understand basic networking .

When two different subnets(say 10.0.0.0/24 and 20.0.0.0/24) wants to communicate among themselves, router is required.

I want to understand in detail why ?

[Brandon Buffin]

In simple terms, the subnet mask (/24 or 255.255.255.0 in this case) breaks the IP address down into network and host portions. It tells the device which devices are local to it's network and which devices are remote. Local traffic can be dropped on the wire for use by local devices. Traffic destined for remote devices will be sent to the default gateway assigned for further routing.

Take a look at the following link for in depth explanation.

http://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13788-3.html

Brandon

[Mark Malone]

Hi Ravi

This question should be posted in the learning network rather than CSC forum , you will probably get a better response and if you search it its most likely been asked already multiple times

https://learningnetwork.cisco.com/welcome

You need a layer 3 device not just a router a layer 3 capable switch can do it too to allow communication between broadcast domains , a layer 2 switch is not capable of this , a broadcast domain is a set of specific devices/hosts in same layer 2 domain a vlan

OSI

layer 1 physical cablong bit rate

layer 2 data ---vlans , data layer mac layer

layer 3 --routing / ip layer

https://en.wikipedia.org/wiki/OSI_model#Description_of_OSI_layers

[Julio E. Moisa]

Hi 

A router or other kind of layer 3 devices (like switch multilayer) is required to connect 2 different vlans or subnets, if we are talking about vlans the procedure is called intervlan routing. Also remember that the gateway address of each subnet is used to allow the communication with different subnets, these gateways are configured on layer 3 devices. 

The switch where the vlans are created will send the information related to the vlan into a frame to the layer 3 device and it will take the decision to route. L2 devices don't have the capability. 

Please verify this link: https://networklessons.com/switching/intervlan-routing/

I hope it is useful  :-)

[Joseph W. Doherty]

We have routers to support a routed network which allows us to avoid a flat network that doesn't scale up beyond "small" networks.  This is especially true with shared media networks.

That's the "why" answer to your question.  For your "detail" part of your question, that can run into multiple books.  For example, unlike IPX, which is also routed, IP wouldn't be able to scale up the way it has if not for how its prefix matching works with summarized addresses, and the way the Internet topology addressing is managed.