You need to have both iDRAC Port card and iDRAC enterprise license if you need dedicated network port for iDRAC. You can refer below link for more details on this [URL] [URL] 
Hello I see folks selling the dedicated IDrac for the Dell T420 Other folks are selling the license it supposedly needs a license.  i read there is a portal (digital locker) for the licenses.  I just wanted to verify if this would work on my two T420s Thanks!!  
Adding Dedicated Idrac T420