16 consecutive rounds. The output of one round feeds to the next. Growing complexity. Each round is carried out with the help of a subkey. There are 16 subkeys, each 48 bits long, and each derived from the 56 bits key used for this encryption.
Each round follows the same computational order: The 64 bits block is divided to a 32 bits left side, and a 32 bits right side. Each round switches the halves. Alas, while the input righthalf becomes the lefthalf in the current round the input lefthalf is acted on before it becomes the right half in the new round. After that the two halves are combined to a new (massaged) 64 bits, as its output, and as input to the next round.
Formally:
L_{i}= R_{i1}
R_{i}= L_{i1} XOR f(R_{i1}, K_{i})
where XOR is the exclusiveor function, Ki
is a 48 bits sub key of the original 56 bits key. The index i reflect the situation after round i=1,2,3,.....16
The "f function is combination of transposition, XOR with key material and substitution, described below.

