usage:
check_find_new_hosts [-v+] [-x] [-s] /dir ip netmask contact

where:
dir is the directory containing the .cfg files (and .skip files if you want to skip certain hosts)
ip is an ip in the range you want to scan
netmask is an integer standing for the netmask eg:24 = 255.255.255.0
contact is the contact group you want new hosts found to answer to.

example:
./check_find_new_hosts -v /etc/nagios/network 192.168.10.129 25 domain-admins


when troubleshooting how this is working use -vv or -vvv from a command line
for more information consult the very very verbose help (may want to | less on a small screen):
./check_find_new_hosts -vvvh

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check_find_new_hosts requires the following (that I know of):

1)	fping is installed at /usr/sbin/fping
2)	traceroute is installed at /usr/sbin/traceroute
3)	Nagios must parse a directory of cfg files, rather than implicitly listing the files eg:

    cfg_dir=/etc/nagios/hosts_hostgroups_and_services
    rather than:
    #cfg_file=/etc/nagios/contactgroups.cfg
    #cfg_file=/etc/nagios/contacts.cfg
    #cfg_file=/etc/nagios/dependencies.cfg
    #cfg_file=/etc/nagios/escalations.cfg
    #cfg_file=/etc/nagios/hostgroups.cfg
    ...
4)	the service generic_service must be defined (the following is what I use):

define service{
	name				generic-service	; The 'name' of this service template, referenced in other service definitions
	active_checks_enabled		1	; Active service checks are enabled
	passive_checks_enabled		1	; Passive service checks are enabled/accepted
	parallelize_check		1	; Active service checks should be parallelized (disabling this can lead to major performance problems)
	obsess_over_service		1	; We should obsess over this service (if necessary)
	check_freshness			0	; Default is to NOT check service 'freshness'
	notifications_enabled		1	; Service notifications are enabled
	event_handler_enabled		1	; Service event handler is enabled
	flap_detection_enabled		1	; Flap detection is enabled
	process_perf_data		1	; Process performance data
	retain_status_information	1	; Retain status information across program restarts
	retain_nonstatus_information	1	; Retain non-status information across program restarts

	register			0	; DONT REGISTER THIS DEFINITION - ITS NOT A REAL SERVICE, JUST A TEMPLATE!
	}

5)	the 24x7 time period should be defined

define timeperiod{
	timeperiod_name	24x7
	alias		24 Hours A Day, 7 Days A Week
	sunday		00:00-24:00
	monday		00:00-24:00
	tuesday		00:00-24:00
	wednesday	00:00-24:00
	thursday	00:00-24:00
	friday		00:00-24:00
	saturday	00:00-24:00
	}

6) a contact group is defined
7) the network that is being looked at can be pinged

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This check would be installed as multiple commands(one for every subnet) and can then be added as a check to whatever host makes the most sense to have it on(I have it as a check on the Nagios host machine)

For example, if you wanted to check the network 192.168.0.x (let's say your servers) and all you host and service definitions (at least all of them for this subnet) are in the directory /etc/nagios/network and the contact group you are using is "admins" you would need the following:

define command{
	command_name	check_find_new_hosts_servers
	command_line	/usr/lib/nagios/plugins/contrib/check_find_new_hosts -v /etc/nagios/network 192.168.0.0 24 admins
	}

if you also wanted to check 192.168.10.[129-255] (how about these are your domain controllers) with the contact group "domain-admins":

define command{
	command_name	check_find_new_hosts_domain
	command_line	/usr/lib/nagios/plugins/contrib/check_find_new_hosts -v /etc/nagios/network 192.168.10.129 25 domain-admins
	}


You would also need services defined to check these:

define service{
	use			generic-service
	host_name		main-server-router
	service_description	FIND_NEW_HOSTS
	is_volatile		0
	check_period		24x7
	max_check_attempts	3
	normal_check_interval	5
	retry_check_interval	1
	contact_groups		admins
	notification_interval	120
	notification_period	24x7
	notification_options	c,r
	check_command		check_find_new_hosts_servers
	}

define service{
	use			generic-service
	host_name		main-domain-controller
	service_description	FIND_NEW_HOSTS
	is_volatile		0
	check_period		24x7
	max_check_attempts	3
	normal_check_interval	5
	retry_check_interval	1
	contact_groups		domain-admins
	notification_interval	120
	notification_period	24x7
	notification_options	c,r
	check_command		check_find_new_hosts_domain
	}

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once run it may generate the following files (so either run it and generate these files or steer away from them):
x.y.cfg where x.y is from the ip address of the host found: 192.168.15.32 would be put in 15.32.cfg
name.cfg where name is from the fdqn: freddy.mybuddy.com would be put in freddy.cfg
nagios_hostgroup_unknown.cfg (expects this file either to not exist or to contain 1 hostgroup):

define hostgroup {
	hostgroup_name		undefined
	alias			Non-Configured
	members			freddy,betsy,bob,...
	}


use "--nowrite --stdout" ( -xs ) as a command line option to write these to stdout instead of to files if you just want to test it or
use -vvx to show what it is doing but don't write any files and don't show the files on stdout