FreeTDS™ converts the servername to an IP address by following the steps below, stopping when it succeeds.
Name lookup sequence [6]
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 inet_addr(3).
Request name-lookup from the operating system via gethostbyname(3) or similar.
If the TDS version and port are not read from freetds.conf, they are derived from the compiled-in defaults and overridden by applicable environment variables.
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 TDSPORT and TDSVER environment variables — and rely on DNS for name resolution.