$ sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg

Error: Could not find any hostgroup matching 'network-printers' (config file '/usr/local/nagios/etc/printers/NC_FrontOffice.cfg', starting on line 10)
Error: Failed to process hostgroup names for host 'NC_FrontOffice' (config file '/usr/local/nagios/etc/printers/NC_FrontOffice.cfg', starting at line 10)
   Error processing object config files!

# OBJECT CONFIGURATION FILE(S)
# These are the object configuration files in which you define hosts,
# host groups, contacts, contact groups, services, etc.
# You can split your object definitions across several config files
# if you wish (as shown below), or keep them all in a single config file.

# You can specify individual object config files as shown below:
cfg_file=/usr/local/nagios/etc/objects/commands.cfg
cfg_file=/usr/local/nagios/etc/objects/contacts.cfg
cfg_file=/usr/local/nagios/etc/objects/timeperiods.cfg
cfg_file=/usr/local/nagios/etc/objects/templates.cfg

# Definitions for monitoring the local (Linux) host
#cfg_file=/usr/local/nagios/etc/objects/localhost.cfg

# Definitions for monitoring a Windows machine
cfg_file=/usr/local/nagios/etc/objects/windows.cfg

# Definitions for monitoring a router/switch
cfg_file=/usr/local/nagios/etc/objects/switch.cfg

# Definitions for monitoring a network printer
# cfg_file=/usr/local/nagios/etc/objects/printer.cfg


# You can also tell Nagios to process all config files (with a .cfg
# extension) in a particular directory by using the cfg_dir
# directive as shown below:

cfg_dir=/usr/local/nagios/etc/servers
cfg_dir=/usr/local/nagios/etc/printers
cfg_dir=/usr/local/nagios/etc/switches
# cfg_dir=/usr/local/nagios/etc/routers

define host{
	use		generic-printer		; Inherit default values from a template
	host_name	NC_FrontOffice 		; The name we're giving to this printer
	alias		NC_FrontOffice 		; A longer name associated with the printer
	address		10.4.0.30		; IP address of the printer
	hostgroups	network-printers 	; Host groups this printer is associated with
	}