Example Microsoft Subnet Question

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Hi All,

I've been reading up on Subnetting and applied what I learnt to the following Microsoft exam example but I don't know how they derived the Network Addresses, may be someone can offer a better explanation. Thanks.


Question:

Your boss asked you to subnet a network in the lab for an upcoming class. He hands you a piece of paper while he's on the phone and it simply says "192.168.10.x/25. 4 subnets." What is the subnet mask and the first address in each subnet?


Correct Answer:

255.255.255.128 / 192.168.10.1, 192.168.10.33, 192.168.10.65, 192.168.10.97

(I got the answer right, based on the subnet - as it was the only one that had 128 in the answer, however, I didn't get the Network Addresses correct)



Explanation given:

Using the network notation /25 indicates you need to use 25 network bits. 255.255.255.0 typically is used for the default subnet mask for Class C networks and uses 24 bits (often noted /24). Therefore, if you add one bit, you change the left-most bit in the right-most octet to 1. This equals 128, creating a subnet mask of 255.255.255.128. The starting addresses (the first assignable IP address) in each subnet would have the right-most octet set to 1, 33, 65, and 97. The network addresses themselves would be 192.168.10.0, 192.168.10.32, 192.168.10.64, and 192.168.10.96.




I don't understand how they derived the number of Hosts, and it's making the assumption that the first network is 192.168.10.0.

I read the following site on Subnetting and used the same principles but didn't get it right. http://www.pantz.org/software/tcpip/subnetchart.html

 

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that subnet mask with 192.168.10.0 gives you:

192.168.10.0-->127 (host range 1-126)
192.168.10.128-->255 (host range 129-254)

To get the First host addresses given as the correct answer you would need a subnet mask of /27 or 255.255.255.224 not /25