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# Subnetting

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Which subnet does host 172.29.187.129 255.255.254.0 belong to?

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Member Posts: 428 ■■■■□□□□□□
You can check it here - https://cidr.xyz

There are also other subnet calculators online you can use. However, if you are preparing for Net+, I suggest you understand the theory behind it.
X year plan: (20XX) OSCP [ ], CCSP [ ]
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Member Posts: 453 ■■■■■■■□□□
edited March 2022
Flip the bits!
Translates to everything to binary. You will see you only have to deal with the last bit in the subnet mask.

172.29.187.129  =  10101100.00011101.10111011.10000001
255.255.255.254 = 11111111.11111111.11111111.11111110

Bits in the IP address that correspond to the "1" bits in the subnet mask cannot change so concentrate on the last octet.

The first 7 bits in the last octet are locked because of the corresponding "1" bits in the mask. Flip the last bit to "0" to get the subnet IP (first ip):
(1000000)1
(1111111)0
-------------
10000000 = 128

Flip the last bit to "1" to get the last IP:
10000001=129

Since this is a /31 for a point to point connection(p2p), you will not see the normal subnet and broadcast IP.

If the subnet mask was a /30 you would have seen four IPs: subnet IP, two usable IPs and a broadcast IP.
(100000)01  = 129
(111111)00 = 252

Flip the last two bits to "0" and you get a subnet ip of .128
10000000

Flip the last two bits to "1" and you get a broadcast ip of 131
10000011

With two usable IP of .129 and .130 (.10000001 and .10000010).

/30 and /31 are used for p2p connections with the /31 being used when IP space is scarce.

Work smarter NOT harder! Semper Gumby!