This is the problem that was randomly generated with my answers:
Mask = 255 . 255 . 255 . 1 1 1 1 1 0 0 0
Convert 233 to bin: 1 1 1 0 1 0 0 1
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Do bit-wise AND: 1 1 1 0 1 0 0 1 (= 232)
Magic = 8
What is the Dotted Decimal Subnet Mask? 255 . 255 . 255 . 248
Find this SubNetwork 205 . 68 . 198 . 232
Find the First useable IP address 205 . 68 . 198 . 233
Find the Broadcast address 205 . 68 . 198 . 239
Find the Last useable IP address 205 . 68 . 198 . 238
What is the Total Number of IPs per Subnet? 32 - 29 = 3 ; 2^{3} = 8
How many useable Hosts Addresses per subnet? 8 - 2 = 6
What is the Next Subnetwork Number? 205 . 68 . 198 . 240
What IP Class is this? Class C
What is the 15th subnetwork number? 205 . 68 . 198 . 112
As you can see, I got number 10 wrong. The correct answer should have been:
10. 205 . 68 . 198 . 120
I counted subnetwork number zero as the first subnetwork. Although some networks do use network number zero, for the puposes of subnetting, never count subnetwork zero! Start counting at the next one up. In this case, 205 . 68 . 198 . 8 would have been subnetwork number 1, making
205 . 68 . 198 . 120 subnetwork number 15, not 205 . 68 . 198 . 112.