URL aliases and bidirectionality I have responsibility for a number of Drupal 6 and Drupal 7 sites and I'm curious about a difference I've noted among them.

URL aliasing is enabled on all of the sites, so that a user who navigates to 'some/readable/path' is directed to 'node/123' but continues to see the 'some/readable/path' URL alias in the browser's address bar.

On most of the sites, if the user requests 'node/123', that URL is displayed in the browser's address bar. On a few of the sites, however, the user who enters 'node/123' sees 'some/readable/path' in the address bar.

I was not aware that this sort of "bidirectional aliasing" was possible; it doesn't seem to be default behavior. Is there a module which provides this bidirectional capability? If it is default behavior, how is it enabled?
By default in Drupal if you have node/7 with an alias of some/readable/path then a user can go to either of those URLs and it will load the page.

For SEO reasons it is considered bad to have the same page viewable at multiple URLs.

For this reason a very large number of drupal sites use the redirect module.

The redirect module forces users to always access a node from the same URL so if a user navigates to node/7 it will redirect them to the aliased URL instead.

It is likely the difference between the sites you are looking at is the redirect module.