We have already discussed the Nagios installation and  nrpe configuration on the previous post  the following demonstrate how to configure client machines to be configured and monitoring,

# vim /usr/local/nagios/etc/objects/host.hostname.cfg

define host{
        use                     linux-server
        host_name         hostname.com
        alias                   hostname
        address             192.168.0.200
        }
define service{
        use                             generic-service
        host_name                 hostname.com
        service_description   Host Status
        check_command       check_nrpe!check_hosts
        }
define service{
       use                               generic-service
        host_name                  hostname.com
        service_description    server Load
        check_command        check_nrpe!check_load
        }
define service{
        use                             generic-service
        host_name                 hostname.com
        service_description    Disk Usage
        check_command        check_nrpe!check_sda2
        }
define service{
        use                             generic-service
        host_name                 hostname.com
        service_description   Http
        check_command       check_nrpe!check_http
        }
define service{
        use                             generic-service
        host_name                 hostname.com
        service_description   MYSQL
        check_command       check_nrpe!check_mysql
        }

Define your host name on host group to be displaying server status on the browser,
 
# vi /usr/local/nagios/etc/objects/localhost.cfg

define hostgroup{
        hostgroup_name   Nagos Servers    ; The name of the hostgroup
        alias                      Remote Servers   ; Long name of the group
        members               localhost,host.hostname.com
        }

Also configure in your nagios.cfg files, should be added the configured path to that file
 
# vim /usr/local/nagios/etc/nagios.cfg

cfg_file=/usr/local/nagios/etc/objects/host.hostname.cfg

Once you have updated these steps check whether the configuration is correct?
 
# /usr/local/nagios/bin/nagios  -v /usr/local/nagios/etc/nagios.cfg

Total Warnings: 0
Total Errors:   0

Things look okay - No serious problems were detected during the pre-flight check

You can restart the Nagios server,
 
# /etc/init.d/nagios restart
Running configuration check...done.
Stopping nagios: .done.
Starting nagios: done.