For formula and validation fields:

• Create a validation rule:
  • Rule Name: Contact_must_be_in_Account_ZIP_Code
  • Operator: AND (return true if both conditions are true)
  • Define the two conditions that, combined, will show the error message:
    • The contact is associated with an account id
    • The contact mailing zip code is different than the account shipping zip code

• • • Hint: Use the API names (MailingPostalCode and Account.ShippingPostalCode) and the <> (Not Equal) operator.
  • Enter an error message for the validation rule.

I have used this formula :

`AND ( NOT( ISBLANK( AccountId ) ), MailingPostalCode <> Account.ShippingPostalCode )`.