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I'm new to networking and I'm trying my best to understand some concepts. Therefore, if routers never existed and we had only layer 2 switches, what would be the problems that arise and cannot be solved by the switch ? Through this question, I'm trying to get to understand for which reasons it was necessary to create the router.

Regards

Debunkar
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    If you didn't have routers or equivalent devices the volumes of traffic just having devices finding each other would overwhelm the network and also the computers. You would also have massive difficulties managing failover and load balancing - just for start. – davidgo May 15 '20 at 10:59
  • I gave 10 seconds of thought for the Spanning Tree Protocol in such an Internet and it already gave me a headache. Routing has been invented to solve problems that can't simply be addressed on layer 2. – Esa Jokinen May 15 '20 at 11:07
  • May be a noob question, but is still certainly worth asking. Don't see the reason for down votes. – Ron Trunk May 15 '20 at 14:40

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I think it's easy to immediately leap to performance/scaling related issues, but I think a more fundamental problem is ownership.

Different organizations want their own networks that they control themselves, L3 and its routing is essentially the solution for interconnecting such networks as necessary.

If you really want a "flat" structure where everyone is connected to everyone, then maybe there can be solutions for the L2 scaling issues, at least up to a point.

Håkan Lindqvist
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Circa 1969, ARPANET was created to get computers to talk to each other, as they spoke entirely different languages. In today's terms, every layer from 1 to 7 was different. This was a problem when someone at MIT wanted to send electronic mail to Berkeley.

Research into packet protocols resulted in Internet Protocol. Named such because it would connect very different networks. Messaging processing computers evolved into routers. Commercial use took off, and the Internet escaped government labs.

Even today, the layers are maintained to separate layer 2 administrative and failure domains. Clouds don't particularly want your bridging loops to bring down their networks, so IP it is.

John Mahowald
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Here's an answer from our sister site Network Engineering:

Imagine you are connected to a bridge with 3 ports. One port is connected to your host; one connects to networks to the west of you, and the last one connects to networks to the east of you.

Now suppose you want to send data to a device with a layer 2 address. How does your bridge know whether to forward the packet to the east or the west? Obviously, your bridge will look up the MAC address in its forwarding table and forward the packet out the proper interface.

Now imagine you are in a global network. Are you going to keep every MAC address in the world on your bridge? What happens as devices are turned on or off? If they move? How will you find unknown devices? Will you flood over the entire globe? You can't keep track of every device on a global network.

In short, layer 3 address provide routing information. Layer 3 address have a hierarchy that conveys location as well as identity. The network portion of an IP address tells you where the host is (topologically), and that information can be summarized, so your PC doesn't need to know how to forward data to every host on the Internet. That ability to scale routing makes a global Internet possible.

Ron Trunk
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Okay my stab at the question:

It all boils down to the term: Collision domain.

If there were no routers then you would need to have all computers connected to the same subnet.

I am assuming all computers are connected through a network of switches instead of hubs due it will limit where a datagram packet is being sent.

For reference:

When a packet is being sent through a hub, that package will be proper gated to all devices connected to the hub and thereby severely affecting the available bandwidth.

That is basically why you have bad WiFi on 802.11 b/g/n networks. Too much noise from your neighbours!

Switches on the other hand limits the traffic to only concern those two devices that are currently speaking to each other, but we still run into problems: Mainly ARP.

Before two computers can talk to each other, they need to find out how to get in contact with each other.

This happens through ARP broadcasting.

The dialogue goes something like this:

192.168.1.5: Hi! I am 192.168.1.5 and my MAC address is aa-bb-cc-dd-ee-ff what is the MAC address of 192.168.1.10?

(All devices listens and memorises the MAC and IP address and one replies).

192.168.1.10: Hi! I am 192.168.1.10 and my MAC address is bb-cc-dd-ee-ff-00. 

(All devices listens and memorises the MAC and IP address).

Now without routers you will get ARP broadcast data packages from all devices connected on the Internet.

That would be a bad thing. Just read up on Distributed Denial of Service attacks.

What happens is you will be flooded with data and unable to reply.

It will basically be a shouting contest between you and the rest of the world. Whoever shout the loudest win - which means you will loose every time.

Also unless you use IPv6, you wont be able to speak with all machines connected today.

There are simply not enough IPv4 addresses available, but that is an entirely different discussion.