Today during a free online CCENT exam I was given the following question:
Which 2 statements best describe IP Address 10.16.3.65/23?
The 2 answers were:
The Broadcast Address of the subnet is 10.16.3.255
The Lowest Host Address in the subnet is 10.16.2.1/23
My questions are, how is this question solved methodically and whats happening to the 3rd octet? It seems to be changing from .3 to .2 when there is only 1 bit in the 3rd octet to play with (NNNNNNNH).
Think of it like this.
Consider the third octet of the /23 mask: 11111110 What does that rightmost 1 represent? It represents the 2s place, so the interval between subnets is 2. Some example subnets would be:
10.16.0.0/23
10.16.2.0/23
10.16.4.0/23
10.16.6.0/23
And so on. Now, the particular IP address in question, 10.16.3.65, falls into the 10.16.2.0/23 subnet. Hence, you just need to look at the subnet itself to determine the answers to your questions.
The broadcast address is the highest address before the next highest subnet. In this case, 10.16.3.255. The lowest host address or the first usable IP is one more than the subnet address. Since the subnet address is 10.16.2.0 the lowest host address is 10.16.2.1.
Don't look on octets in this questions. IPv4 is 32 bits. /23 mean first 23 bits is network address, other bits is host address.
