In Linux, How to connect through SSH over LAN and WAN, What makes the difference from LAN and WAN, How is it achieved over WAN i.e. over internet?

One of the ways is to configure your server side router to allow SSH. i.e. Through PORT FORWARDING

Usually within a LAN environment, it is through the terminal you can login into the remote desktop through the SSH command and the IP address.

#ssh 192.168.0.101

#ssh username@ipaddress

You need to install SSH Server in the system you wish to login i.e. Remote System

How to install SSH Server on Linux?

Ubuntu

#sudo apt-get install openssh-server

Redhat

#yum install openssh-server

Arch

#sudo pacman -S openssh

After installing SSH Server you need to start the sshd.

Within the LAN

To login remote SSH, you need remote login password.

ssh user@ipaddress

#ssh user@192.168.0.101

Before you login, make sure 'sshd' service is started in the remote system. By default SSH server start listening on 'port 22'. After installing SSH Server, you can check the status with the below command.

#sudo systemctl  start sshd

#sudo systemctl status sshd

#sudo systemctl restart sshd

Without need to start the ssh server manually, if the service is enabled it will start on every reboot. The same way you can disable the service too.

#sudo systemctl enable sshd

#sudo systemctl disable sshd

How to find your IP Address? Through 'ifconfig' command.
To use this command 'ifconfig' you need to install 'net-tools'

#ifconfig

Ubuntu

#sudo apt-get install net-tools

arch

#sudo pacman -S net-tools

Now you can login to remote shell within the LAN, you can achieve the same over the internet using PORT FORWARDING.

For PORT FORWARDING login to your server side router and open firewall settings, in the IPv4 and IPv6 add new rules for SSH. Configurations for routers may differ so I am giving an example of a router configuration.