In Class C network, you have 8 bits in host portion and 24 bits in network portion.
Subnetting is to create subnets of an exisitng network by borrowing bits from host portion. So you can create subnets by borrowing 1-6 bits from host portion because you have to leave atleast 2 bits in host portion to have atleast 2^2-2=2 hosts.
so IP address 184.108.40.206/25 will have 255.255.255.128 subnet mask.
That is only if you're not using IP Subnet Zero. This was typical of older routing protocols such as Rip V1. With any classless routing protocol using ip subnet zero, you can absolutely have a /25 subnet. If you used a /25 without subnet zero, that means you could not use the first or the last subnet and since there are only two subnets within a /25, there would be none left to use. That's where the borrowing at least 2 bits came from. Hope this helps.
If you remember before ip subnet-zero, you lost the upper and lower subnets, so Cisco used to say that you had to borrow at least 2 to cover the 2 subnets lost and left you with 2 usable. This is an old rule. You can use /25.
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