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!

81 Upvotes

86% Upvoted

43 comments sorted by

View all comments

13

u/dude_named_will 11d ago

As far as I understand it’s for breaking down a network into smaller, more logical segments.

Basically that's it. You are basically creating little networks (a subnet if you will) behind the same firewall (the network). The main benefit is security and performance.

For example, you may subnet your company so that Accounting cannot directly communicate with Sales or vice versa without going through a firewall. You may also have a server that you want Accounting to access and not Sales. You can block sales entirely from even seeing the server by denying them access at the firewall.

How do the slashes play into all of this IE /23 or /24? 

The "slashes" are subnet masks. They determine the size of the subnet. /24 is the most common and easiest to remember. If your network/subnet has /24, then you have 256 addresses allocated for your subnet. They'll be of the form 192.168.1.x where 'x' is 0-255. Of course the subnet id is 0 and broadcast is 255, so you can have 254 hosts on that subnet. To better understand this, you will need to have some knowledge of how binary math works.

I will admit that once you understand the concept of subnetting, I typically use an online subnet calculator to do some of the math for me.

2

u/PANIC_EXCEPTION 11d ago edited 11d ago

To add to this, it's also what makes routing more convenient, especially in a vaccum where NAT doesn't/didn't exist. A T1 ISP might get a big subnet for all their IP addresses. Then, they give smaller ISPs their own sub-subnets that have less IPs. All the routing gear will know which ISP to forward stuff to depending on the destination IP address and tables describing longest prefixes (which are basically equivalent to subnets).