Displaying ints lead leading zeros

I'm using a textbox with a MaskedEditExtender to only allow users to enter numbers in a textbox (for a ZIP code).
This works fine except when the ZIP code is lead by zeros - eg 00820.
When I get the value from the database (where its stored as an int) and out it in the textbox its shown as 820 not 00820.
How do I make sure the appropriate number of leading zeros are shown ?
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