nbtscan is a command-line NetBIOS scanner for Windows that is SUPER fast, it scans for open NetBIOS nameservers on a local or remote TCP/IP network, and this is the first step in the finding of open shares.
It is based on the functionality of the standard Windows tool nbtstat, but it operates on a range of addresses instead of just one.
What is nbtscan?
NETBIOS is commonly known as the Windows “Network Neighborhood” protocol, and (among other things), it provides a name service that listens on UDP port 137.