How Can We Have Otional Fields In A Table?

Hello to all,

i want to make a table in order to keep the data of a hard disk.
e.g. create table disk{
capacity integer,
name varchar(50),
ports integer,
buffer char(1));

For example i want this hard disk to have these fields and i would like from the user to type me only the capacity because it is going to be a required field and the other fields(name,ports,buffer) to be optional!

How do i declare optional fields in a create table command?
and then how do i declare/define these fields in insert too?

Thanks, in advance!