host_url
relies on using the host name of the computer rather than an IP
address and typically works on local networks, although this is not always
guaranteed. If unsuccessful, substitute an IPv4 or IPv6 address in place of
the hostname.
local_url
generates a random URL for the platform's default inter-process
communications transport: abstract Unix domain sockets on Linux, Unix domain
sockets on MacOS, Solaris and other POSIX platforms, and named pipes on
Windows.