I'm afraid that because you've provided only a code snippet rather than a full MWE (minimum working example), some vital information -- the width of the text block of your document -- is not available. I've thus had to make some (possibly incorrect) assumptions: A4 paper size, 1-inch margins, two-column mode.
Below is a succession of tables that address various issues you raise. I've deliberately not centered the tables to make it easier to compare the widths of the tables. (In a real document, you should of course encase each tabular environment inside \begin{table} \caption{...} \label{...} \centering [tabular stuff] \end{table} statements.) First, there's a horizontal rule indicating the width of the text block. In the first table, the missing vertical lines are restored by adding a couple of & symbols. Obviously, the table is (much) too wide and won't fit in the available space.
In the second table, the widths of columns 2 and 3 are reduced significantly by moving the unit designations to separate lines. (This should also help reduce any confusion over whether the (s) after Delay is a unit designation or the plural-s to Delay.) In the third table, the width is reduced further, succeeding in making the table fit in the text block. Importantly, all vertical lines are eliminated, allowing some more (horizontal) space saving; in addition, the look of the horizontal lines and the spacing above and below the horizontal lines is improved by replacing the basic LaTeX \hline macro with \toprule, \midrule, and \bottomrule macros.

\documentclass[a4paper,twocolumn]{article}
\usepackage[margin=1in]{geometry}
\usepackage{multirow,booktabs}
\setlength\parindent{0pt}
\begin{document}
Width of text block, two-column mode:
\hrule
\bigskip
(a) Restore the missing vertical lines:
\smallskip
\begin{tabular}{|c|c|c|c|}
\hline
Topology & Average Delay(s) & Average Bandwidth(kbps) & Hosts \\
\hline
Low & 0.0994 & 2441.85 & \multirow{2}{*}{64} \\
High & 0.1424 & 2515.77 & \\
\hline
Low & 0.2522 & 1798.60 & \multirow{2}{*}{128} \\
High & 0.1113 & 2123.17 & \\
\hline
\end{tabular}
\bigskip
(b) Reduce table width by moving units designations
to separate line:
\smallskip
\begin{tabular}{|c|c|c|c|}
\hline
Topology & Average Delay & Average Bandwidth & Hosts \\
& (s) & (kbps) & \\
\hline
Low & 0.0994 & 2441.85 & \multirow{2}{*}{64} \\
High & 0.1424 & 2515.77 & \\
\hline
Low & 0.2522 & 1798.60 & \multirow{2}{*}{128} \\
High & 0.1113 & 2123.17 & \\
\hline
\end{tabular}
\bigskip
(c) Better horizontal lines, no vertical lines, reduce table
width some more:
\smallskip
\begin{tabular}{@{} cccc @{}}
\toprule
Topology & Avg.\ Delay & Avg.\ Bandwidth & Hosts \\
& (s) & (kbps) & \\
\midrule
Low & 0.0994 & 2441.85 & \multirow{2}{*}{64} \\
High & 0.1424 & 2515.77 & \\[2ex]
Low & 0.2522 & 1798.60 & \multirow{2}{*}{128} \\
High & 0.1113 & 2123.17 & \\
\bottomrule
\end{tabular}
\end{document}
booktabsmanual. – You Jul 18 '12 at 09:10excel2latexthat make producing tables a lot easier. Most of those tools are discussed in this question: http://tex.stackexchange.com/questions/49414/comprehensive-list-of-tools-that-simplify-the-generation-of-latex-tables – matth Jul 18 '12 at 11:53