[Tutor] question about ascii and bytes

[nephish]

ok, so i have sparce documentation from a server that i need to make a
socket connection with. 

the server wants to get info a certain way. Each message has to start
with an ascii "STX" (literally the three letters, not the standart ascii
'STX') then it has to have another four bytes that give the length of
the message, then the message itself, then end with an ascii "ENX" 
(again,the three letters, not the ascii character "ETX" ).

So here is where the biggest gap in my understanding is... how do i send
ascii characters ? 

the docs say that if the password is not 24 bytes long, the other bytes
must be filled with null values. How do i do that ? What is a null
value?

python strings are ascii already , right ? would this be as simple as
simply passing the string to the socket.send('pythonAsciiString') ?

thanks for any tips or links for documentation . 

sk