Enable Anonymous Access at Web Application in SharePoint 2010

To Enable Anonymous Access for a Web Application, follow below steps

Step 1: Open Central Administration, click on Manage web applications under Application Management.

 Step 2: Select the Web Application for which you need to Enable anonymous access and click the Authentication providers from the Ribbon. 

Step 3: From the popup, click on Default zone. 

Step 4: Select the option Enable anonymous access and the save the changes. Now the Anonymous access is enabled at Web application level, to enable at Site collection level see EnableAnonymous Access at Site Collection