There's a very neat interactive map of BGP nodes/peerings on dn42 at http://nixnodes.net/dn42/graph/
Edit: Looking at the IPv6 view, I see no reason why OnionCat and GarlicCat couldn't peer. It's just that gateways would be needed. And that could chew up humongous bandwidth.
% dig whois.dn42 @resolver.nic.dn42 any +short
172.22.0.43
fd42:d42:d42:43::
as well as some ACME implementation with a CA that is constrained to the .dn42 domain and the allocated IP space. % openssl x509 -in /etc/ssl/certs/dn42_Root_Authority_CA.pem -noout -text
[...]
X509v3 Name Constraints:
Permitted:
DNS:.dn42
IP:172.20.0.0/255.252.0.0
IP:FD42:0:0:0:0:0:0:0/FFFF:0:0:0:0:0:0:0
[...]
So there's quite some stuff to do and learn about.