After Mac DHCP (dynamic IP) move on 1 Mar 2017, doing scp on Discover to transfer files from Discover to my local machine is not working. The solution is:

(1) visit http://whatismyip.com to find my IP address (which is 198.120.226.91)
(2) on my local machine, use the command "nslookup 198.120.226.91" to find a hostname associated with the IP. What I got is
    Server:        128.183.10.134 
    Address:    128.183.10.134#53 
    91.226.120.198.in-addr.arpa    name = gs6101-fzengm1.ndc.nasa.gov. 
So the host name is gs6101-fzengm1.ndc.nasa.gov. 
(3) on Discover, connect to my local machine using the command "ssh gs6101-fzengm1.ndc.nasa.gov", then the hostname "gs6101-fzengm1.ndc.nasa.gov" will be automathically added to "Discover:/home/fzeng/.ssh/known_hosts"
(4) now I can transfer files using the command "scp filenames gs6101-fzengm1.ndc.nasa.gov:/../."
(What I used to do was "scp filenames gs6101-fzengm1:/../.", but now I need to add "ndc.nasa.gov" after "gs6101-fzengm1")