nmap -sP x.x.x.x show host up, but mtr --report x.x.x.x provide no results. Why?

I can't provide the real IP..

Thanks!!
mylinuxserver# mtr --report xx.xx.xx.xx
HOST: mylinuxserver Loss% Snt Last Avg Best Wrst StDev
mylinuxserver#

mylinuxserver# nmap -sP xx.xx.xx.xx -PN

Starting Nmap 5.21 ( http://nmap.org ) at 2014-07-29 13:28 GMT
Nmap scan report for xx.xx.xx.xx
Host is up.
Nmap done: 1 IP address (1 host up) scanned in 13.00 seconds

Well,.. what do you get if you run an nmap traceroute?

mylinuxserver# nmap -sn --traceroute xx.xx.xx.xx -PN

Starting Nmap 5.21 ( http://nmap.org ) at 2014-07-29 15:52 GMT
Failed to find device eth2 which was referenced in /proc/net/route
Nmap scan report for xx.xx.xx.xx
Host is up.

TRACEROUTE (using proto 1/icmp)
HOP RTT ADDRESS
1 ... 30

Nmap done: 1 IP address (1 host up) scanned in 22.15 seconds

Right. So there isn't a traceroute to see. That is why mtr isn't reporting anything.

You are using the `-PN` flag, which means "Treat this host as up, without sending any packets to confirm." That is why Nmap thinks the host is up. You can see more info with `--reason`, which will show "Host is up, received user-set."

If you remove the `-PN` option and Nmap finds the address up, then it is because it is responding to one of the default host discovery probes. I'm not sure what mtr uses, but I'd guess it's an ICMP Echo Request (a.k.a. Ping). Nmap uses that as well as 2 different TCP probes and an ICMP Timestamp Request probe by default, if you run it with root permissions. Otherwise, it uses TCP connections to port 80 and 443 to determine if the host is up.