\documentclass[]{article}

%opening
\title{Ip Checksum Example}
\author{Scot Anderson, Ph.D.}

\begin{document}

\maketitle

\section{Example}

\begin{tabular}{rrrrrr}
	&  & $0001$ & $0001$ &  & $(17)$ \\
	$+$ &  & $1111$ & $0111$ &  & $(-8)$ \\ \cline{1-4}\cline{6-6}
	& $1$ & $0000$ & $1000$ &  &  \\
	&  & \multicolumn{1}{l}{$\hookrightarrow $} & $+1$ &  &  \\ \cline{3-4}
	&  & $0000$ & $1001$ & $=$ & $(9)$ \\
	\multicolumn{6}{l}{Hence the checksum is:} \\
	&  & $1111$ & $0110$ &  &
\end{tabular}

\bigskip

If we sum up the original two numbers with the checksum we get:

\begin{tabular}{rrrr}
	&  & $0001$ & $0001$ \\
	&  & $1111$ & $0111$ \\
	$+$ &  & $1111$ & $0110$ \\ \cline{1-4}
	&  & $11111$ & $1110$%
\end{tabular}
			
Of courxse with the carry over we have:\ $1111~1111$ which is $-0.$ 

\end{document}
