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I am studying for my CCNA and my current topic is subnetting.

I understand how subnetting works and how to subnet (calculate the network IP, the amount of hosts, broadcast, etc). However, I can't seem to figure out when or why would we use all that.

Let's say that we have a network of 192.168.1.0/24. There are 200 hosts and we would like to split them into 4 separate subnets, where each subnet would have 50 hosts.

If we want subnets that would serve 50 hosts, we would use the /26 prefix. /26 prefix would create 4 subnets that would serve 62 hosts, but it's the closest to 50. The network addresses would be 192.168.1.0/26, 192.168.1.64/26, 192.168.1.128/26 and 192.168.1.192/26 enter image description here

I can understand the purpose of subnetting, if it's related to public IPs, because if we make subnets for a specific amount of hosts, we would prevent IP addresses from being wasted.

But why would we want to subnet private IPs like this? We can't really run out of/waste private IP addresses, can we? Couldn't we just use 192.168.1.0/24 for the first subnet, 192.168.2.0/24 for the second subnet, 192.168.3.0/24 for the third subnet and 192.168.4.0/24 for the last subnet, instead of picking a specific prefix and calculating the network addresses?

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I hope I didn't make anything confusing. Please ask if something is unclear.

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    But why would we want to subnet private IPs like this? We can't really run out of/waste private IP addresses, can we? Oh yes, we can
    – Ron Trunk
    Mar 3, 2022 at 16:16
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    "We can't really run out of/waste private IP addresses, can we?" That depends on the company size. For example, where I work has such a problem because networks were not properly sized, (people assuming they want /24 for everything) and there is a lot of wasted addressing. Also, for point-to-point links, you really want to use /30 or /31 networks to prevent ping pong attacks.
    – Ron Maupin
    Mar 3, 2022 at 16:17
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    I don't understand the difference you are making. In the first example, you are subnetting 192.168.1/24 into /26 subnets. In the second example, you are subnetting 192.168/21 into /24 subnets. Why do you find the first one unacceptable and the second one acceptable, even though you are doing exactly the same thing in both (except a slight difference in the sizes of the networks)? Mar 5, 2022 at 15:52

2 Answers 2

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But why would we want to subnet private IPs like this? We can't really run out of/waste private IP addresses, can we?

You must be imagining small networks. My clients have tens of thousands of users with hundreds of remote sites. Each one has several subnets. Address conservation is indeed a problem.

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    Same here - 230k+ users, 40 main sites and hundreds of locations. 10.0.0.0/8 is rather small...
    – Zac67
    Mar 3, 2022 at 16:26
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Another way to look at this is from the design perspective of a large campus. A typical internal private IP addressing design would be 10.B.V.0/24. Where B is the building number and V is a VLAN within that building. This also gives you the ability to summarize all the routes in a building by having the building advertise 10.B.0.0/16. This "hides" any localized outages, such as a switch getting shut off, and keeps the routing tables stable throughout the campus.

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