Decentralized network 42 (also known as dn42) is a decentralized peer-to-peer network built using VPNs and software/hardware BGP routers.
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While other darknets try to establish anonymity for their participants, that is not what dn42 aims for. It is a network to explore routing technologies used in the Internet and tries to establish direct non-NAT-ed connections between the members.
The network is not fully meshed. Dn42 hardly uses physical links between the single networks but virtual ones. These virtual links use the Internet as transport but are logically independent from it, you can treat a virtual link more or less like a fiber or copper line. Everybody has a VPN connection to one or more participants. The connections are bound to a public IP address. If you decide to use GRE or SIT the diactc (Diac24 Tunnel Controller) updates the public IP address of the connection endpoint on IP changes. OpenVPN is capable of doing this without external help. Over the VPN connection BGP is used for inter AS routing and by default OSPF for intra AS routing, but participants may obviously use any protocol they see fit for the purposes of routing inside of their own AS.
Address space
Network address space for IPv4 consists of private subnets: 172.20.0.0/14 is the main subnet. Note that other private address ranges may also be announced in dn42, as the network is interconnected with other similar projects. Most notably, ChaosVPN uses 172.31.0.0/16 and parts of 10.0.0.0/8.
For IPv6, both Unique Local Address (ULA, the IPv6 equivalent of private address range) and Globally routable addresses are used, with a preference for ULA.
AS numbers
In order to use BGP, even in a private environment, Autonomous System Numbers are needed. dn42 uses several private or reserved AS numbers ranges, including 64512 to 64855 and 76100 to 76199. In June 2014, dn42 started using a new private range, 4242420000 to 4242429999, part of larger private range defined by RFC 6996.
BGP routers
Hardware used on dn42 consists of routers, which aren't always a cheap solution. There are mainly four implementations for BGP on default hardware. The most common in dn42 is Quagga, a fork of the no longer maintained GNU Zebra, which runs on most Unices from Linux over *BSD to OpenSolaris. Also used in dn42 is OpenBGPD which runs on OpenBSD. There are also the "enterprise class" XORP and the simplistic BIRD, however those are not very common in dn42. There were also some machines running JunOS, the Juniper Operating system, which is used with professional routing hardware.