SharePoint prompts user to login

I do not work with SharePoint often, but I know that I had this problem before.  Do not remember and cannot find how to fix it.

I have SharePoint 2013 setup.  In Alternate Access Mapping, I have:

Default: http://sp_servername
Internet: http://intranet.domain.com

As far as I remember the problem is related to this setting.  If I go to http://sp_servername, everything looks good.  If I go to http://intranet.domain.com, it asks user to login.  Every time.

Can someone help?  We have to use http://intranet.domain.com in the office and when we are outside of the office.

Thank you.

Thank you. Eric.