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while running:
nmap -n -PN -sT -T4 -p 445,139 10.0.40.0/24 i get:

Interesting ports on 10.0.40.6:
PORT STATE SERVICE
139/tcp filtered netbios-ssn
445/tcp filtered microsoft-ds

if I specifically scan a single ip

nmap -n -PN -sT -T4 -p 445,139 10.0.40.6
Interesting ports on 10.0.40.6:
PORT STATE SERVICE
139/tcp open netbios-ssn
445/tcp open microsoft-ds

why there is a difference?

Note:
if I omit the -sT from the sub net scan I get: PORT STATE SERVICE
139/tcp open netbios-ssn
445/tcp open microsoft-ds

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They should be identical. I am thinking that maybe it has something to do with latency/scan timeout. Are the results the same if you try a different scan type, and a slower scan time?

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thanks @gerryk , i thought so to , but isn't nmap suppose to handle slow connection , running the command from the same subnet being tested result in open connection found – modi Apr 12 '11 at 10:51

-sT is an ACK scan. Well it is in some way trying to stay under a remote log's radar... So the router might detect that you are doing something weird and is filtering you out. Perhaps you could either drop (-sT) from your options or use a SYN scan(-sS).

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