How do I set access permissions per field? What I need to do

I have some user account fields that should be administrator-only (read/write/edit).  For example, the IP address used to register.

So, I want to restrict access to administrators.

Current state of field permissions in Drupal 8 (as of Feb 19, 2016)


In Drupal 7, I used the Field Permissions module.  However, a Drupal 8 version is not currently being worked on by the maintainers.
There is a discussion on putting field permissions in Core, but this has been going on for seven years and is marked as "postponed."
You're stuck doing this in code, but the Examples module includes a field_permissions example module.