|FreeTDS User Guide: A Guide to Installing, Configuring, and Running FreeTDS|
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FreeTDS converts the servername to an IP address by following the steps below, stopping when it succeeds.
Name lookup sequence 
Find servername in freetds.conf. If a section with that name exists, use the hostname, port, and TDS version specified therein.
Attempt to convert servername to an IP address with
Request name-lookup from the operating system via
gethostbyname(3) or similar.
As you can see, if most of your servers use the same TDS version and answer to the default port, then you don't need to list them all in freetds.conf. You can simply compile in the right defaults — or set the
TDSVER environment variables — and rely on DNS for name resolution.
This description applies to DB-Library and CT-Library. ODBC lookup is different.