We'll now learn how to reverse engineer subnets. If you're given a 192.168.1.127 address with a mask of 255.255.255.224, what is the subnet address? First, find the increment, which in this case is 64. Now, round the address to the next lower 32 (if it's not a multiple of 32). The subnet address is then 192.168.1.96. The network range is then 192.168.1.97 to 192.168.1.126 where .96 is the subnet address and .127 is the broadcast.
Given an host IP 172.16.68.65 with the mask of /28. The default gateway is .62. Is the gateway address valid? Now let's find the subnet the host belongs to. Since /28 has an increment of 16, then he belongs to the 172.16.68.64 subnet.
Now let's look at the default gateway. It has an address of .62. Round it down to the next 16 and you'll find that it belongs to the .48 subnet. Now, since the host is in the .64 subnet and the gateway is in the .48 subnet, then they are not in the same subnet and therefore the gateway cannot be used by the host.
This sort of questions are typical in CCNA examinations and should be mastered.
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