String
rrToString() {
	StringBuffer sb = new StringBuffer();
	sb.append(byteArrayToString(address, true));
	if (subAddress != null) {
		sb.append(" ");
		sb.append(byteArrayToString(subAddress, true));
	}
	return sb.toString();
}
Returns the ISDN subaddress, or null if there is none.
src/main/java/core/org/xbill/DNS/ISDNRecord.java