You can easily disable comments by default on new pages and custom post types, with the ability to individually set them on a page or post basis.
By default, WordPress provides you two options:
- You can have them active by default.
- You can either disable comments and trackbacks by default for all pages and posts.
WordPress doesn’t provides you any specific setting that allows comments and trackbacks to be active by default for posts, while disabling them on pages or any other post type but using No Page Comment plugin you can have these options also. Simply install, activate it then visit its options from Dashboard->Settings->No Page Comment.