You can't directly connect to an SSH server if it's in a private network or behind a NAT or firewall. An SSH could also be configured only to allow connection from a specific host or network and deny connection from everywhere else. Some possible solutions to this are connecting to the specific host or network via a VPN, or using a proxy.

You can configure PuTTY to tunnel its connection through a proxy. The supported proxy types for PuTTY are HTTP, SOCKS(4/5), Telnet and Local.

Related: How to create SOCKS proxy on Windows

Steps to connect to SSH server using PuTTY via proxy:

  1. Launch PuTTY.
  2. Go to Connection → Proxy from the Category menu in the sidebar.
  3. Choose the type of proxy that you are to use.
  4. Enter the IP address or hostname of the server along with the port that the proxy service is listening to.

  5. Set if whether or not you want the DNS resolution to be done on the proxy server or locally.
  6. Enter proxy credentials if required.
  7. Go to session from the Category menu.
  8. Enter connection details such as the hostname or the IP address of the server that you want to connect to, local to the proxy server.
  9. Click on the Open button to start the connection.