You can't directly connect to an SSH server if it's in a private network or behind a NAT or firewall. An SSH server 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.

Step-by-step video guide:

Steps to connect to SSH server using PuTTY via proxy:

  1. Launch PuTTY.
  2. Go to the Proxy option from the sidebar menu.
  3. Choose the type of proxy that you are connecting to.
  4. Enter the IP address or hostname and the port of the proxy server.
  5. Choose whether or not to route DNS queries via the proxy.
  6. Enter credentials for the proxy server if required.
  7. Go to the Session option from the menu.
  8. Enter connection details of the SSH server that you want to connect to.
  9. Click on the Open button to start the connection.
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