how to get valid URL in combination of text and field content in VIEWS Field HOSTNAME returns IP Adress.
I have to overwrite the field output .
Is there a way to modify this with an given URL like this:
'www.ip-tracker.org/locator/ip-lookup.php?ip=' and adding the field content of HOSTNAME to get an valid URL?
Yes you can use the Rewrite results option on your Field settings. Use the Output this field as a link option and enter only the URL with the field token replacement pattern at the end of it. 

You can look up the token under Replacement patterns under the Rewrite results option. 

If you need some more control over the link HTML you can use the Rewrite the output of this field where you construct the entire link HTML &lt;a href="....