n2n is a layer-two peer-to-peer virtual private network (VPN)
which allows users to exploit features typical of P2P
applications at network instead of application level.
This means that users can gain native IP visibility (e.g. two
PCs belonging to the same n2n network can ping each other)
and be reachable with the same network IP address regardless
of the network where they currently belong.
In a nutshell, as OpenVPN moved SSL from application (e.g. used
to implement the https protocol) to network protocol, n2n moves
P2P from application to network level.

The main n2n design features are:

- An n2n is an encrypted layer two private network based on a
  P2P protocol.
- Encryption is performed on edge nodes using open protocols
  with user-defined encryption keys: you control your security
  without delegating it to companies as it happens with Skype
  or Hamachi.
- Each n2n user can simultaneously belong to multiple networks
  (a.k.a. communities).
- Ability to cross NAT and firewalls in the reverse traffic
  direction (i.e. from outside to inside) so that n2n nodes are
  reachable even if running on a private network. Firewalls no
  longer are an obstacle to direct communications at IP level.
- n2n networks are not meant to be self-contained: it is possible
  to route traffic across n2n and non-n2n networks.