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I've been reading about subnetting on a few stack exchange sites and others that I found online but the majority of posts seem to contain outdated responses such as class networks etc.

So to my knowledge, subnetting is used to make traffic easier to direct to the correct place which lowers congestion. This could be used by having a different subnet mask for each department in a business and if department A wanted to talk to department A then it would stay within the local subnet, otherwise it would look at the mask and determine where to send it instead of broadcasting it to everyone.

With switches, where traffic is only sent to the correct destination and not broadcast to everyone, is subnetting still needed? If so why?

Simon P
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    Switching and routing occur at two different layers of the OSI network model; Layer 2 and Layer 3, respectively. So switching isn't related to routing, whether you're using switches or hubs. This doesn't give you a detailed explanation of switching, routing, subnetting or the use of VLAN's because, frankly, that would be a very long answer. – joeqwerty Nov 05 '15 at 17:58
  • Hopefully you're using a modern Linux or windows that uses IPv6 – Jim B Nov 05 '15 at 20:24

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Yes. When you replace a hub with a switch, you are only making changes to layers 1 and 2 (physical hardware and MAC addressing). No changes to layer 3 (IP routing and subnetting) are being made.

user3150166
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Subnets make routing tables smaller at the expense of forcing IP address reassignment when a computer roams.

This was a good trade off in the past, when networks grew slowly, the resources consumed by routing table entries were scarce special purpose hardware (a TCAM line in the router) and few hosts roamed to different physical locations.

It's much less important today. Networks grow faster, and the administrative efforts to manage subnets are a bigger factor than in the past. Many networks have only one or a few subnets, often covering big chunks of the RFC1918 address space, and VLANs are often used to restrict visibility of hosts with respect to each other within a single subnet instead.

Will Angley
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