SSH connection refused

Problem

I SSH to another Linux computer and get this error:

$ ssh joe@172.20.128.100
ssh: connect to host 172.20.128.100 port 22: Connection refused

Solution

Typically, there is no complicated reason of permissions why this failed. Since the OpenSSH client is almost always installed by default on Linux computers, people assume that the OpenSSH server is also installed. It is not and it gives this error!

Install the OpenSSH server on the host and you should be able to SSH to it:

$ sudo apt install openssh-server

Tried with: Ubuntu 14.04