Suggested enhancement: IP address documented within Marketo for single IP addresses
As a short term solution: Using the dropdown within the email editor to select an IP address for instances with multiple IP addresses, I suggest offering the same dropdown for instances that have only one dedicated IP address.
As a long term solution: Document the actual IP addresses (non-editable) within Admin/Email.
Reasoning: Avoid putting the documentation of the IP addresses into a random Word document that is filed away or eventually deleted.
You should enter this in the Idea section, so that the Product Management is notified of this. And also other users can vote