Well, bottom line is, if you have a big network, and you have no monitor on it now, then somebody better get to work. Unless you don't care how long it takes to find a fix a problem with your network. Now let's see, how does this switch get to the router? Which port? Ummm, ahh heck, just setup nagios and get it over with. It checks our network, every switch, every router, every cable/port used to connect one to the other, every eth0 on a host and how it connects to a switch, the cpu loads, temp, fan RPM's power supplies, and on, and on, and on,.....
Of course, it didn't do this by itself. I've put plenty of work into it. And I can guarentee that I didn't just tell it "look at the 10.0.0.0" net and see what's working. No, I had to find out for myself, how my nagios pc connects to a switch, how that switch connects to another, how that switch connects to a router. I then looked at every switch in the company and figured out what port was used to connect from A to B, to router, etc. It's not easy, but is is worth it? You tell me. It takes me 5 minutes to discover exactly which cable has been cut/unplugged/etc and it would take you how long?
If you could not ping to your router, is it your eth0 cable/card, the switch you are plugged into, perhaps the other switch that daisy chains to the router you can't ping? You don't know, unless you actually ping the switches first. Oh, but wait, you don't remember the ips of the switches or even which switches are in between you and the router. Ahh, heck, call someone who knows how the network is cabled up and let them fix it. NOT!!! This is NOT how you want to fix your network. Nagios will not only tell you what is broken, but you will learn your entire network layout due to the need for you to configure nagios correctly.
Get my point?