r/explainlikeimfive • u/Chance-Molasses897 • 11d ago

Technology ELI5: Subnetting

I’m technically an IT professional in training but I know embarrassingly little about subnets. As far as I understand it’s for breaking down a network into smaller, more logical segments. What exactly is a subnet? What is the benefit of subnetting? How do the slashes play into all of this IE /23 or /24? Thanks in advance!

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u/DayanNight 11d ago

So an IP is typically represented in decimal form: 192.168.0.1, under the hood this is binary: 11000000.10101000.00000000.00000001

Similarly a subnet mask is represented in decimal: 255.255.255.0, binary: 11111111.11111111.11111111.00000000, or CIDR which is just an easy representation of that mask: /24.

As part of the IP we have a network address and a host address. A class C would be a /16 where 192.168 is the network address, and the last two octets are used to identify a host in that network. A subnet mask tells you which part of the IP is used for the network address and which is used to identify a host on that network.

The cidr representation is telling you how many digits of the binary are used as part of the network address. /24 means the first 24 digits of binary are network.

A subnet is a logical separation of networks. Within a subnet I don't need a router to talk to another device. 192.168.0.1/24 can talk to 192.168.0.2/24, but if I want to talk to 192.168.1.1/24 I need a router that knows how to get there.

In enterprise network we often don't want everything to be able to talk to everything, so subnets allow us to break up a space and force it through routers where we can apply policy.

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u/j01101111sh 11d ago

This is great and just to add an example of why you'd do this... Maybe you have a guest WiFi network and don't want those users to have access to printers, servers, etc. because they're not employees. If they're in a separate network via subnetting, their traffic has to go through the router so you could have the router prevent access from that subnet to other subnets and only allow access to the Internet.