Ubuntu 11.04 (Natty) has stopped supporting, but can use old-releases source, first back up the source list file:
sudo cp/etc/apt/sources.list/etc/apt/sources.list.backup
sudo gedit/etc/apt/sources.list
Replace the contents with the following:
Deb Http://old-releases.ubuntu.com/ubuntu/natty main restricted universe multiverse
Deb Http://old-releases.ubuntu.com/ubuntu/natty-security main restricted universe multiverse
Deb Http://old-releases.ubuntu.com/ubuntu/natty-updates main restricted universe multiverse
Deb http://old-releases.ubuntu.com/ubuntu/natty-proposed main restricted universe multiverse
Deb Http://old-releases.ubuntu.com/ubuntu/natty-backports main restricted universe multiverse
DEB-SRC Http://old-releases.ubuntu.com/ubuntu/natty main restricted universe multiverse
DEB-SRC http://old-releases.ubuntu.com/ubuntu/natty-security main restricted universe multiverse
DEB-SRC http://old-releases.ubuntu.com/ubuntu/natty-updates main restricted universe multiverse
DEB-SRC http://old-releases.ubuntu.com/ubuntu/natty-proposed main restricted universe multiverse
DEB-SRC http://old-releases.ubuntu.com/ubuntu/natty-backports main restricted universe multiverse
Overload the source list for the configuration to take effect:
sudo apt-get update
To install the SSH server and client:
sudo apt-get install openssh-server openssh-client
Ubuntu 11.04 (Natty) has stopped supporting but can use the old-releases source