If you want to handle external links differently than internal links (Example: Open all external links in new window) without additional coding, I recommend you to install the External Links module :"External Links is a small module used to differentiate between internal and external links. Using jQuery, it will find all external links on a page and add an external icon indicating it will take you offsite or a mail icon for mailto: links."
Important Note: Using the options "Add icon to external links " and "Add icon to mailto links " can create layout/CSS problems (i.p. conflicts with other modules/elements). Example: Various social media related modules; because the icon is also added to the social media links (being external links), the alignment is often disturbed.