Commands to install iftop on Centos:

Pre-req install of libraries:

yum -y install libpcap libpcap-devel ncurses ncurses-devel

Download and Install iftop:

wget http://www.ex-parrot.com/pdw/iftop/download/iftop-0.17.tar.gz
tar -zxvf iftop-0.17.tar.gz
cd iftop-0.17
./configure
make
make install

To make use of iftop run the command:

iftop