\(\displaystyle \bold{A.} \ 200.10.5.56\)
\(\displaystyle \bold{B.} \ 200.10.5.32\)
\(\displaystyle \bold{C.} \ 200.10.5.64\)
\(\displaystyle \bold{D.} \ 200.10.5.0\)
The answer is \(\displaystyle \bold{C.} \ 200.10.5.64\).
But why?
The answer to this question lies on your attack strategy. I am as a professional in bungee jumping, the first thing I think of when I see an \(\displaystyle \text{IP}\) is the \(\displaystyle \bold{block size}\)!
What is the block size?
The block size tells you the number of maximum \(\displaystyle \text{IP}\)s in each subnetwork. It also helps you identify what is the first \(\displaystyle \text{IP}\) (Network Address) in each subnetwork.
How to find the \(\displaystyle \bold{block size}\)?
You look at this \(\displaystyle \text{IP}\) \(\displaystyle 200.10.5.68/28\).
And you focus on \(\displaystyle /28\).
This number tells you how many network bits are used.
In other words:
\(\displaystyle /28 = 11111111\textcolor{red}{\bold{.}}11111111\textcolor{red}{\bold{.}}11111111\textcolor{red}{\bold{.}}11110000\)
You see above we have \(\displaystyle 28\) ones \(\displaystyle \longrightarrow\) network bits.
Four \(\displaystyle (4)\) bits are left for the host bits. These \(\displaystyle \textcolor{darkblue}{4}\) bits can tell us the \(\displaystyle \bold{block size}\).
\(\displaystyle \bold{Block size}\): \(\displaystyle 2^{\textcolor{darkblue}{4}} = 16\)
Now we know that each subnetwork has \(\displaystyle 16 \ \text{IP}\)s.
The first subnetwork always starts at zero.
\(\displaystyle \bold{1}^{\text{st}}\) subnetwork address: \(\displaystyle 200.10.5.0\)
\(\displaystyle \bold{2}^{\text{nd}}\) subnetwork address: \(\displaystyle 200.10.5.16\)
\(\displaystyle \bold{3}^{\text{rd}}\) subnetwork address: \(\displaystyle 200.10.5.32\)
\(\displaystyle \bold{4}^{\text{th}}\) subnetwork address: \(\displaystyle 200.10.5.48\)
\(\displaystyle \bold{5}^{\text{th}}\) subnetwork address: \(\displaystyle \textcolor{indigo}{200.10.5.64} \longrightarrow\) Our \(\displaystyle \textcolor{red}{\text{IP}}\): \(\displaystyle 200.10.5.\textcolor{indigo}{68}\) is in this subnetwork.
\(\displaystyle \bold{6}^{\text{th}}\) subnetwork address: \(\displaystyle 200.10.5.80\)
And so on. You just start with \(\displaystyle 0\) and then add the \(\displaystyle \bold{block size} = 16\) to go the next subnetwork address.
