Quick and easisest way:
wget http://apt.sw.be/redhat/el5/en/x86_64/rpmforge/RPMS//rpmforge-release-0.3.6-1.el5.rf.x86_64.rpm
rpm -Uvh rpmforge-release-0.3.6-1.el5.rf.x86_64.rpm
yum install ntop

# Get yum ready
yum clean all
yum update

# Install Development Tools
yum groupinstall “Development Tools”

# Install more ntop Dependencies
yum install libpcap-devel libpcap

# Install RDD tools dependencies
yum install cairo-devel libxml2-devel pango-devel pango libpng-devel freetype freetype-devel libart_lgpl-devel

# Download, compile and install RDDTools
wget http://oss.oetiker.ch/rrdtool/pub/rrdtool-1.4.5.tar.gz
tar xvzf rrdtool-1.4.5.tar.gz
cd rrdtool-1.4.5
./configure
make
make install

# Download, compile and Install GeopIP
wget http://geolite.maxmind.com/download/geoip/api/c/GeoIP.tar.gz
tar xvzf GeoIP.tar.gz
cd GeoIP-1.4.6/
./configure
./make
./make install

NOTE: config at: /usr/local/etc/GeoIP.conf

#Download, compile and install nTop 4.0.1
wget http://cdnetworks-us-1.dl.sourceforge.net/project/ntop/ntop/ntop-4.0.1/ntop-4.0.1.tar.gz
tar xvzf ntop4.0.1.tar.gz
cd ntop-4.0.1/
autoconf
./configure –with-rrd-home=/opt/rrdtool-1.4.4/
make
make install

# Add ntop user and directory permissions.
useradd -M -s /sbin/nologin -r ntop
chown -R ntop:root /usr/local/var/ntop/
chown -R ntop:ntop /usr/local/share/ntop/

# Set ntop password
ntop -A

# To start ntop on boot
nano /etc/rc.local # and add: /usr/local/bin/ntop -i “eth0,eth1″ -d -L -u ntop -P /usr/local/
you have multiple interface (eth0, eth1 and so on), start ntop as follows:

# To start ntop
/usr/bin/ntop -i “eth0″ -d -L -u ntop -P /var/ntop –skip-version-check –use-syslog=daemon
htpasswd -c /etc/httpd/.htpasswd segredes

Where,

* -i “eth0,eth1″ : Specifies the network interface or interfaces to be used by ntop for network monitoring. Here you are monitoring eth0 and eth1.
* -d : Run ntop as a daemon.
* -L : Send all log messages to the system log (/var/log/messages) instead of screen.
* -u ntop : Start ntop as ntop user
* -P /usr/local/var/ntop : Specify where ntop stores database files. You may need to backup database as part of your disaster recovery program.
* –skip-version-check : By default, ntop accesses a remote file to periodically check if the most current version is running. This option disables that check.
* –use-syslog=daemon : Use syslog daemon.

By default ntop listen on 3000 port. You can view ntop stats by visiting following url:

http://localhost:3000/