NMAP Arp ping fails discovery

[Christian Castelli <voodoo81people () gmail com>]

Hi everybody,
I'm new in the list, I'm a web developer involved in writing a simple
network monitoring solution. The objectives are only to test
availability of nodes in LANs and their traffic statistics (rx/tx got
from SNMP requests) and for host discovery I'm using nmap.
My hosting provider has four machines which can see each other and
which respond to ping inside the LAN.
When I launch *nmap -sn -n 10.26.12.16/28* without sudo, it finds all
my four running machines, but if I use it with sudo, arp ping request
seems to fail (verbose option told me) and the scanning finds only the
host from which I run nmap.
FYI I'm running nmap from an opened ssh shell via VPN connection,
anyway I don't believe this might cause any problem though.
It's interesting that if I use arp-scan tool it can finds everything.
What could be the causes of this behaviour? I also specified the
interface for both tools.
Thank you in advance.

-- 
Chris
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