To get a list of SQL Server instances currently installed in your network, execute either of the below commands
Exec xp_cmdshell 'SQLCMD /L'
Exec xp_cmdshell 'OSQL /L'
If you want to populate a text file with the results then, open the command prompt (Go to Run, Type cmd and click ok) and type this command and hit enter
SQLCMD /L > C:\ListOfServers.txt
This command will populate the text file ListOfServers.txt with the results obtained from SQLCMD /L