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The only thing to remember is that normally if you create a host without 1 or more services, you will get a warning regarding that. Normally, if I'm just monitoring a host's up-status, I toss a pingcheck in for the same host.
So, hosts are virtual or physical devices, services are just that...hardware, software, services, and other types of checks.
Sorry cause im doing nagios for a school assignment. So is it okay to ignore the warning and just monitor a host using ping without declaring a service for it?
You should have at least one service for each host, even if it's just a ping check. You could change the host check to something else if you don't want two checks pinging the host. You also could actually put all your services under a single host if you wanted to, regardless of whether or not they're located on the same box/address. It would make things pretty unmanageable in a production environment though.
Normally I use simple check_host_alive for hosts, and then use more complicated or targeted checks for services. Unless, of course, upstatus is all you care about. In my production environment, I normally make sure to have as few warnings as possible, but I still occasionally use a serviceless host. 2 pings is too much for a low-cost host when you're monitoring hundreds of hosts and hundreds upon hundreds of services in a high-availability environment.