Rails 3 server on Vagrant box drops port from URL

Running a rails 3 server with bind to and IP address:

bundle exec rails server -b 0.0.0.0

    => Booting Unicorn
    => Rails 3.2.21 application starting in development on http://0.0.0.0:3000
    => Call with -d to detach
    => Ctrl-C to shutdown server
    listening on addr=0.0.0.0:3000 fd=16
    worker=0 spawning...
    master process ready
    worker=0 spawned pid=10459
    worker=0 ready

And it's available in the host machine at url 127.0.0.1:3000.

Some of the hyperlinks "work" (leading to 127.0.0.1:3000/a/path/view), but other hyperlinks and all FORM actions drop the port from the URL (127.0.0.1/a/path/view = 404 not found).

I'm new to rails and not sure if this is due to a problematic inconsistency in the way the Rails application is set up, the Vagrant configuration or something else.

Can anyone recommend a good place to start poking around for answers?