LwIP IPv4/IPv6 stacks

From lwIP Wiki

With actual code design, you need to choose between IPv4 or IPv6, since the stack can't support the both IP versions in the same time. This choice have to be done at build time, since some functions, macros and types are defined with the same names/identifiers, but in different files.

Note that IPv6 is still in a very experimental state, and that no real support is actually done... If you would like to help us improve our IPv6 support, we'd love to have your help! :)