To make the links other posts more domain name independent, it would be nice to have internal links.
By default Drupal does not have this feature, but it is possible by using PathFilter.
PathFilter translate a relative internal path to a full URL. This module integrates with the Drupal HTML Filter.
The project page of PathFilter is here: http://drupal.org/project/pathfilter
Download the latest version and put the module into your Drupal modules directory (/sites/all/modules).
The activate and configure the module, do the following steps:
I tested this with CKEditor, but I think the base is the same for other editors.
Attention! Path Filter and a multi-domain Drupal installation is not recommendable. Sometimes an internal link will be redirected to an another domain. This results in broken links en that is not nice for the visitors and not good for the SEO ranking.