Padding of Original Message in MD5 what should I do if my original message is greater than 64 bits? what is the process of padding in MD5?

As described in Internet Security: Cryptographic Principles, Algorithms and Protocols, page 138, the second step in MD5 is to append the length of the message:



Can someone help me understand exactly what happens when the message length is greater than $2^{64}$ bits?
If the original message length $N$ is $2^{64}$ bits or longer, you encode the value $N \bmod 2^{64}$ in the final padding block, rather than just $N$.  Alternatively, you can just always encode $N \bmod 2^{64}$; if $N &lt; 2^{64}$, then they're the same value.

Now, this distinction between $N$ and $N \bmod 2^{64}$ doesn't happen at 64 bits, it's at $2^{64}= 18,446,744,073,709,551,626$ bits, or about 2000000 Terabytes.  No one has ever hashed a message that long with MD5; even if we assume an MD5 compression function can be computed in 1nsec (massively overoptimistic), it'd take over a year to hash a message that large (and parallelism doesn't help).