"VPN tunnels are generally considered as something
mysterious and everybody who mentions them, is "cool" :-) However,
there is nothing to be afraid about, in the fact principle of
Virtual Private Network is very simple.
"Data in IP networks are exchanged in packets. Information about
destination and a source of the packet can be found in the packet's
header. The actual user data are carried as a payload. Let's
imagine that ssh client wants to talk to ssh server over the
Virtual Private Network. Packets leaving a host's network interface
located on the local network are sent with a destination port
number 22. When this packet reaches a VPN tunnel it is encapsulated
into the VPN packet where original packet is now treated as a
payload. If the VPN server listens on the port 443, the VPN packet
will carry a destination port 443.
"When looking at the previous figure it is apparent that this
kind of network data transfer over the VPN is a waste of transfer
rate because original packet has a smaller payload space just
because it needs to fit into the VPN tunnel packet. In VPN
analogical sense this can be considered as a drawback."