192.168.100.0
host=25? [25 => 2^5,32]
32 - 5 = /27
subnet/network = 2^n = 2^5 = 32 ( 0 ~ 31 )
host = 2^n - 2 = 32-2 = 30
usable = 192.168.100.0 ~ 192.168.100.31
1st IP = 192.168.100.1
Last Ip= 192.168.100.30
subnet mask = 255.255.255.224
................................................
32 + 32 = 64 (- 1) = 63
................................................
64 + 32 = 96 (- 1) = 95
................................................
96 + 32 = 128 (- 1)= 127
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R1 - R2/28172.31.1.64172.31.1.65172.31. 172.31.1.255.255.255.240
R2 - R3/28172.31.1.80172.31.1.81172.31.1.172.31.1.255.255.255.240
R3 - R4/28172.31.1.96172.31.1.97172.31.1.172.31.1.255.255.255.240