When you use a column with NOT NULL then it's not possible to add NULL to the column.
I guess you have other columns as well, like:
Create TABLE Products
(
[ProdID] [int] IDENTITY(1,1) NOT NULL,
[Col2] int NULL,
[Col3] int NOT NULL,
[Col4] int NULL
)
When you want to insert a record to this table dont mention the [ProdID] column, it will be filled automaticly because its a identity.
Example:
INSERT INTO Products ([Col2],[Col3],[Col4]) VALUES (1,2,NULL)