ORA-27101: Shared memory realm does not exist

ORA-27101: Shared memory realm does not exist
Cause: Unable to locate shared memory realm
Action: Verify that the realm is accessible

Usually occurs when trying to start an instance and you do not have enough memory or your memory offset is not such that you can run a large SGA.

My work around on this problem

Cause 1)
This error occurs when Oracle Home is not set properly.
Solution)
Check the Oracle Home is set properly. Check the registry and your Oracle installation product have same path

Cause 2)
Error occurs when Oracle SID is not set properly.
Solution)
Check the Oracle SID is set properly. Compare the OracleServices property have the SID and the SID given in your registry both are same.
SID is case sensitive so be careful.

Cause 3)
Sometimes it may so happen that the drive (C: or D:) you have install the Oracle product/database have not define the virtual memory.
Solution)
It needed to define the virtual memory where you have installed the Oracle product/database.

Solution)
Another Solution is that change

SQLNET.AUTHENTICATION_SERVICES=(NTS)
to
SQLNET.AUTHENTICATION_SERVICES=(NONE)

In sqlnet.ora file and restart your database service.