GNU/Linux Desktop Survival Guide
by Graham Williams
Versions of SSH
A Finnish company developed the original ssh but copyright and patent issues lead to the development of the free https://packages.debian.org/search?suite=default§ion=all&arch=any&searchon=names&keywords=OpenSSHOpenSSH.
There are two major (and slightly incompatible) versions of ssh: 1.X and 2.X. From a 2.X client you will not be able to connect to a 1.X server. Connecting to an OpenSSH 2.X server from any version of the client is supported though.