How can I set default value for each column in a Database?

I know I can set it on add_column command but I am looking for a way to
set the default when creating the Table. See below code, how can I set
each of them a default value.

def self.up
    create_table :bay_infos, :primary_key => :IP do |t|
      t.string :HostName
      t.string :IP
      t.string :Description
      t.string :user_name
      t.string :password

      t.timestamps
    end
  end

I know I can set it on add_column command but I am looking for a way to
set the default when creating the Table. See below code, how can I set
each of them a default value.

http://api.rubyonrails.com/classes/ActiveRecord/ConnectionAdapters/TableDefinition.html#M001735

Fred