Eli5 – Subnet Mask

Question

Hello, i know this question has been asked numerous times, but it never managed to truly explain it, and it’s quite frustrating.

So far i understand that a Subnet Mask is used to divide an internal IP address into a Network Portion and a Host Portion. I’ll make the SOHO example because that’s what i’m used to, never saw other networks.

My internal IP is [192.168.1.1](https://192.168.1.1) and the subnet mask would then be [255.255.255.0](https://255.255.255.0).

This would mean that only 254 hosts are possible in my network, right? Since the 1 is the Default Gateway and the last would be the Broadcast Domain.

I still don’t get the point in having a mask.

It is my understanding that when forwarding a frame, the gateway would AND the 2 things:

11111111.11111111.11111111.00000000 &

11000000.10101000.00000001.00000001

the result would be

11000000.10101000.00000001.00000000 ([192.168.1.0](https://192.168.1.0))

It’s all fine and dandy but… i don’t get why? Is this somewhat like an IF statement?

e.g. IF the first 24 bits are 192.168.1, THEN it’s inside this network? But why the extra mask then?

Also because with that logic, all internal IP addresses would then become [192.168.1.0](https://192.168.1.0)

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The IP address range 192.168.1 is already private, why do i need another set of 32 bits for the subnet mask? 192.168.1 is already my inside network, so… it’s obvious that 1 would be my default gateway, i would be 2 and so on…

Is the subnet mask just a way to “make it obvious” to the router?

I really don’t get the sense of it, probably looking at it all wrong.

Thanks in advance

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EDIT: Does it mean that in the routing table, my private IP is also associated to a subnet host? E.g. [255.255.255.4](https://255.255.255.4)? Because running ipconfig i don’t recall seeing my host address.

In: Technology

Answer 1

Because it’s a SUBnet… Meaning you could have more than one subnets on a network. You need to specify the range for subnets so you can treat them differently without having ip conflicts. Determining a subnet range allows for you to know what net a given ip address belongs to. A mask is one syntax to do that. Another is via Cidr IP ranges.