integrated authentication only work when I use ip address

I have a problem with a accessing sharepoint web site using integrated
authenication,
If i use http://ip address it works fine.
If I add a entry to the host file of the client machine ip address
sitename it works fine

But if i try to access it via http://servername or
http://servername.domain.com authenication fails.

I have tried every possible combination with adding the site to local
intranet zone with no luck. This seems to be a DNS problem I am
assuming because adding a entry to the host file works.

Any help will be greatly apprecitated

Thanks in advance

Re: integrated authentication only work when I use ip address

On one of the clients open a command prompt and run

>nslookup

And then type

>servername

and then

>servername.domain.com

check if your dns server holds the IP address - if not you have an easy
fix just add the required A record to DNS for servername in the
domain.com zone.
This would not be an authentication problem though just a name
resolution issue so maybe I am missing something?