I was scanning a machine from inside the same LAN which is running and up to date Windows XP Home Edition which I suspect may be running some malware.
The target machine has an up to date antivirus running (AVG free), but has no other firewall than the one which Windows provides, which provides an option to configure the processes allowed or restricted to open connections but does not provide any option to configure specific ports being open/closed.
One of the scan commands used is this one:
nmap -p 1-65535 -v hostname
And this is part of the output:
Not shown: 65528 filtered ports PORT STATE SERVICE 139/tcp open netbios-ssn 445/tcp open microsoft-ds 2869/tcp closed icslap 4041/tcp closed unknown 12216/tcp closed unknown 16881/tcp closed unknown 23590/tcp closed unknown
I have googled and read thoroughly about closed/filtered differences (and I think that I understood it) but, since there's no firewall configured to block these ports running between me and the other machine nor inside it, this is actually confusing me a lot.
So, does anyone have any clue about what may be causing these 5 apparently random ports appear as closed while the other 65528 appear as filtered?