Simple REST request to create nodes
CCK fields are always stored as an array of fields, even if the actual field is limited to only a single value.

So you it should be something like:

'field_ip_address' =&gt; array(array('value' =&gt; '1.2.3.4'))

Note that in Drupal 7, field values are additionally grouped by the language, which is 'und' (LANGUAGE_NONE) if the field is language-agnostic. So it would be something like this:

'field_ip_address' =&gt; array('und' =&gt; array(array('value' =&gt; '1.2.3.4')))

Copied from my comment, so that the question can be marked as fixed :)