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I didn't get what ports does Nmap scan. Does Nmap start from zero or one when scan subnet or all the ports?

  • 0/24 is it 0-255 or is it 1-255
  • -p is it 0-65535 or is it 1-65535
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Connecting to port 0 is not possible, see

https://unix.stackexchange.com/a/180500/85039

It is a special port allowing programs to obtain random port for outgoing connection, and should not be used for incoming connections

Therefore, use -p 1-65535 for nmap, especially if you are using TCP or UDP scans

As for /24 notation that is entirely different thing: it is called CIDR notation and used to match range of hosts on network. For instance, 192.168.0.0/24 will specify all addresses between 192.168.0.0 to 192.168.0.255

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  • but i can scan all ports in nmap using the command -p 0-65535 – Ven Ven Jan 2 '20 at 19:55
  • so /24 start with zero not one ? – Ven Ven Jan 2 '20 at 20:06
  • @VenVen You can use -p 0-65535. But trying to scan port 0 is pointless - it's never open to outside world, only to programs on the host itself. – Sergiy Kolodyazhnyy Jan 3 '20 at 6:48
  • As for /24, yes, address range starts at 0 and goes to 255. That's 256 hosts in total theoretically. But remember that in practice 192.168.0.255 is broadcast address for the whole 192.168.0.0/24 network. Typically it's not used for a host and ping utility for example will throw an error, but it is perfectly valid address. On a different subnet other than /24, it could be a perfectly valid host. See also stackoverflow.com/a/14915309/3701431 and unix.stackexchange.com/a/407395/85039 – Sergiy Kolodyazhnyy Jan 3 '20 at 6:48
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Nmap can scan all TCP UDP ports. You must to know subnetting and network knowledge. 0 port is not a standart port. For subnet if you use 24 subnetmask you will not scan 192.168.0.0 address because 24 subnetmask have a useable range for hosts. 0.0 is out of subnet useable scope. You can learn from here: http://www.steves-internet-guide.com/subnetting-subnet-masks-explained/

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