We have a 11g set up on 2008 R2 box and can connect fine. We just installed Oracle 12c on Windows Azure VM and we can not get connected. We tested port 1521 to see if it was open and it is.
We are trying Direct connect and have SID checked.
We are getting ORA-12537: TNS: connection closed.
Anyone seen this before?
FYI - Toad 9.7.X came out before 11gR2, so that version of toad could not be tested with 11gR2 or 12c - which are the only two officially supported versions of Oracle currently. So you really need to upgrade as that version of Toad is now 6 years old (Sep 2008). For example one possible error one might get with such an old version of Toad is that it may not recognize the newer Oracle client DLL’s and issue errors messages. You’re really in gray area using such old version of software - and it limits what we can do to assist …
I can connect to Oracle 12c on Azure using SqlPlus.
Thanks Bert for the quickly reply. I will check with Accounting Dept and see if they will release some funds for an upgrade…
If you are current on maintenance upgrades are free - so check to see that first before spending any money 
Just an FYI…I was able to connect my 9.7 Toad using Direct Connect to a 12C DB using an old 10g client with no issue.
Are you using the same client setup to connect to 12c as you did 11g?
Can you tnsping the DB from Toad?
There are a myriad of issues that can cause an ORA-12537 so it can be very difficult to diagnose.
I was able to connect 9.7 to Oracle 12c as well, but I had to tweek my sqlnet.ora file.
I changed TCP.VALIDNODE_CHECKING = YES
to NO.
Now, I don’t know if that opens up some security holes or not, but it did seem to fix my problem.