md5sum

[terry]

I think I was told by a respondent to the list if this MD5sum and an ISO 
disk  image in the same location, maybe a download folder some thing 
like this; md5sum  ubuntu xxxxxx   xxx.iso and I would get the hash of 
the disc to compare with on line so I don't waste a cd if the hash is 
incorrect? I've not had much success with it. More so with  openssl 
dgst  - md5 /path to iso./
Also. I saw where I could do this to get the hashes from a burned disk 
in the cdrom by md5sum /media/sr0 to see if the disc is ok.
Also, which technique is used  md5sum  or sha256sum?, which I found both 
listed on the web sight?
Which one is ubuntu's preference?
Thank you.