Direct cable connect two computers.lapttop rj45 to desktop dp9 how is it done?

Well what is dp9. The only easy way is to use a CAT5 crossover cable from NIC to NIC, RJ45 to RJ45. Now you can go from parallel port to parallel port but you need a special cable and it's SLOW.

If the other PC doesn't have a network card put one in, they can be bought for 15-20 buck.

That's going to be tough unless you come up with a way to add a port on at least one side of the equation.

Does the laptop have USB? Parallel?
Desktop have Parallel? Can you add a network adapter (NIC) to it, even temporarily?

What vintage desktop doesn't have at least parallel?

Theoretically there is probably a method but if you're working with serial communications the data stream will be S-L-O-W regardless of adapter/connection type. Parallel-to-parallel isn't bad but large data transfers are still a problem.

RJ-45 to RJ-45 is your best bet for speed of course.

You are thinking of DB9, not DP9.

But basically, you can't do that. You either need both to have a serial cable, or both to have a NIC.

the laptop only has rj45 and the other computer has only dp9 serial port

BUY A STRIGHT RJ45 CABLE CONFIGURATION FROM RADIO SHAK CONNECT IT TO BOTH COMPUTERS AND RUN NETWORK CONNECTION WIZARD FROM START > ALL PROGRAMS > ACCESSORIES > COMMUNICATION > NETWORK CONNECTION WIZARD. AND JUST PROVIDE IT WITH THE INFO NEEDED. THERES OPTIONS THERE CHOOSE THE THIS U THINK FITS YOUR COMPUTER. ITS VERY EASY. YOU CAN DO IT