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
It limits broadcast traffic. You have 255 devices on one subnet. The switches etc. only have to transmit their broadcast traffic between those 256 devices. (by the way, .0 is a perfectly valid IP to use itself!).
Larger subnets increases the amount of devices and hence the amount of broadcast traffic. Broadcast traffic amplifies itself at the switch – you send one packet, but to send that out, the switches have to send 256 packets to different devices. It sounds tiny, but in any large network it quickly gets out of hand and impacts everyone. You want to reduce the size of, and the number of, broadcast packets at all times.
And actually 192.168.x.x is the private address. You could have 65536 devices. You can use them all. But most people less than 256 devices, so it’s a waste.
It’s a measure to make things manageable, memorable, leave room for other things (e.g. you might want your CCTV cameras on one IP range and your home network on another so that compromise of one doesn’t automatically lead to access to the other – more often done with VLANS in corporate networks but it’s a good idea to have different VLANs have different subnets, so that if they do ever get accidentally configured, they don’t affect each other), and reduce broadcast traffic.
If you want to see your broadcast traffic, run Wireshark on your local wired network (may not work properly on some wireless setups – you need “promiscuous” mode). You’ll see stuff pinging back and forth all the time. Printers advertising their presence, every device on your network enquiring about the physical address of every other device, etc.
256 devices talking to 255 other devices = a lot of broadcast.
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