What is the difference between a BEQUEATH connection and an IPC connection?

A bequeath connection

runs on your local host

bypasses the listener

the protocol creates the server process for you directly

An IPC (Inter-Process Communication) connection

will use the native protocol on each OS, but uses the generic term "IPC" for all of them

can only be used when the Client and Server reside on the same host

can only be used by having the Client connect through the Oracle Listener

the Database Listener must be configured to listen on an IPC endpoint

the listener spawns the server process for you

Example setup:

Listener.ora
 LISTENER =
   (DESCRIPTION_LIST =
     (DESCRIPTION =
       (ADDRESS = (PROTOCOL = TCP)(HOST = myserver)(PORT = 1521))
       (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
     )
   )

tnsnames.ora:

proddb01_ipc =
   (DESCRIPTION =
     (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
     (CONNECT_DATA =
       (SERVER = DEDICATED)
       (SERVICE_NAME = proddb01)
     )
   )

Connect to your database locally:

sqlplus /nolog
SQL*Plus: Release 11.2.0.4.0. Production on Thu Jan 22 15:35:44 2015

Copyright (c) 1982,2013, Oracle. All rights reserved.

SQL> connect scott/tiger
Connected.

From another window, create another session:

sqlplus /nolog
SQL*Plus: Release 11.2.0.4.0. Production on Thu Jan 22 15:35:44 2015

Copyright (c) 1982,2013, Oracle. All rights reserved.

SQL> connect scott/tiger@proddb01_ipc
Connected.

Check the connections and their types:

SELECT S.SID, S.OSUSER,S.PROGRAM,S.USERNAME,S.MACHINE, SCI.NETWORK_SERVICE_BANNER,S.LOGON_TIME,S.STATUS
 FROM V$SESSION S INNER JOIN V$SESSION_CONNECT_INFO SCI
 ON S.SID = SCI.SID
 WHERE S.USERNAME = UPPER('scott')
 AND SCI.NETWORK_SERVICE_BANNER LIKE '%IPC%'
 OR  SCI.NETWORK_SERVICE_BANNER LIKE INITCAP('%BEQ%')
 AND S.TYPE <> 'BACKGROUND'
 ORDER BY LOGON_TIME;

And here is the output. Notice how the first session (.. / as sysdba) results in a Bequeath session, while the other one ( ...@proddb_ipc) results in a session using IPC:

SID	OSUSER	PROGRAM	USERNAME	MACHINE	NETWORK_SERVICE_BANNER	LOGON_TIME	STATUS
9	oracle	sqlplus@myserver.mydomain.com (TNS V1-V3)	SCOTT	myserver.mydomain.com	Oracle Bequeath NT Protocol Adapter for Linux: Version 11.2.0.4.0 - Production	22.01.2015 15:35:49	INACTIVE
1160	oracle	sqlplus@myserver.mydomain.com (TNS V1-V3)	SCOTT	myserver.mydomain.com	Unix Domain Socket IPC NT Protocol Adaptor for Linux: Version 11.2.0.4.0 - Production	22.01.2015 15:40:44	INACTIVE