plz suggest me the query

plz tel me that : suppose that i made a table in sql server 2005 an
there is a column name "department id"..... i want that who so ever
enters the value in this column shoul prefix "DEP" and then 3nos......
it means that the format should be like this "DEP***" WHERE * ARE THE
NOS........ OR is ther any way so that it wil automatically prefix
"DEP"...otherwise it wil throw an error........

CREATE TABLE [dbo].[Departments](
[DepartmentID] [nchar](6) NULL
) ON [PRIMARY]

GO

ALTER TABLE [dbo].[Departments] WITH CHECK ADD CONSTRAINT
[CK_Departments] CHECK (([DepartmentID] like 'DEP[0-9][0-9][0-9]'))
GO

ALTER TABLE [dbo].[Departments] CHECK CONSTRAINT [CK_Departments]
GO

CREATE TABLE Foobar
( ..
department_id CHAR(6) NOT NULL
CHECK (department_id LIKE 'DEP[0-9][0-9][0-9]'),