Specification Internal and external number range?

Hi,

Did someone used Internal and external number range more than 12 char.

I know Standard sap ESN_SUBID will allow user to add upto 12, but i need more than that, its our client requirement.

External like A to zzzzzzzzzzzz External check box checked.

Internal like 000000000001 to 999999999999 for internal.

What we need to do if you need to have 18 char or more than that?