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
The subnet mask is mostly used in the routing table. The routing table consists of a network IP address, a subnet mask, an optional gateway and a network interface. Whenever your computer have a network package to send out it will check if the outgoing address and the network mask equals the network address, if true then it will send the package to the gateway on the specified interface, or if no gateway is set then send the package directly to the recipient on that interface. The routing table is sorted by network mask so you can have multiple overlapping routes and it will pick the most specific route. This may seam quite trivial for single computers as you tend to have only two entries in the routing table, one for the local network and one for the Internet. However for more complex routers with multiple networks and multiple interfaces, most of the interfaces connecting to other routers with their own list of networks, you can see how this works. Some routers have hundreds of thousands of entries and the simplicity of the routing table with the subnet mask for easy lookup means that they can still handle vast amounts of traffic.
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