In Oracle this identifier is called a tnsname and is usually defined in a file called tnsnames.ora. That file lists the tnsnames and the host, db name (SID), and port that are needed to connect.

For example: when I connect to our development oracle database I use "DBI:Oracle:dev" as the database name. In our tnsnames file on the machine I'm working on there is an entry defining "dev" as a database called "MWH" located on the host "cosmora01d" and port "1521".

Update:

You are wanting to research Ingres vnodes aka virtual nodes. In the cited syntax DBI:Ingres:thatnode::thisdb it is 'thatnode' which is the vnode name. Whenever you want to reference a remote database you add the vnode name to the database name to form "vnode::dbname" ala DBI:Ingres:vnode::thatdb.

Hopefully mentioning 'vnode' was enough to jog your memory. All the information about how to access the remote database is somehow associated with a name when you create a vnode using that name.

If not sufficiently jogged you'll need to wander around trying to find information about your local utility program that creates new vnode definitions. I've seen mentions of a 'netu.exe' on Windows (perhaps the utility for an older version?) and a 'netutil' utility elsewhere. I've also seen references to a 'VisualDBA'. These are supposed to allow you to create a new vnode name and supply information describing the remote server.

Google searchs on "Ingres vnode netutil create" and the like may find you more clues. I found at least one user utility manual at ftp://ftp.ca.com/CAproducts/ingres/docs/Ingres_20/net.pdf with docs for other versions found under ftp://ftp.ca.com/CAproducts/ingres/docs/
Good luck!

The following should work fine, assuming Ingres is anything like MySQL, postgresql, etc (the IP you posted is really quite funny, I've never seen a 5-part IP):

$dbh = DBI->connect('dbi:Ingres:mydb;host=192.168.50.0.2');
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