thank you for your help !
I try 10.25.100.27:0 and 10.25.100.27 (alone)
it's the same result (error window)
nmap says :
root@debian7-origine:/home/user# nmap 10.25.100.27 -A
Starting Nmap 6.00 ( http://nmap.org
) at 2013-11-06 16:05 CET
Nmap scan report for 10.25.100.27
Host is up (0.0011s latency).
All 1000 scanned ports on 10.25.100.27 are closed
MAC Address: B8:27:EB:00:88:02 (Raspberry Pi Foundation)
Too many fingerprints match this host to give specific OS details
Network Distance: 1 hop
HOP RTT ADDRESS
1 1.07 ms 10.25.100.27
OS and Service detection performed. Please report any incorrect results at http://nmap.org/submit/
Nmap done: 1 IP address (1 host up) scanned in 1.96 seconds
I ask myself if VNC is launched on the Pi...
In the tty2 logged as root ps => no vnc running...