Dear all ,

I have two laptops , i wanna copy the hard disk from one laptop to the other using the existing network .

thanks in advance

You don't need to use dd command over the network, dd command is not used usually for coping files, it has more advanced options than just coping files "converting and formatting the file to be copied "please see the man pages for dd, for coping or moving files use "cp" or "mv" commands instead.
Now for coping files over network, you can use the "scp" or "rcp" command and of-course "scp" is recommended for security reasons, please "man scp" for details.
Another way to do this is to share your the file system via network and the most obvious candidates here are NFS and SAMBA, googling on them yields to dozens of tutorials and HOWTOs on how to setup a NFS or SMB share.
Please decide what do you want to do want to do and I'm sure every body here will help you

Salaam