Address of the local mock server if it's running, or null otherwise.
This will be non-null between calls to run and stop. The address is a physical IP address, e.g. <code class="literal">127.0.0.1 </code>.
This should not normally need to be passed into client code under test, unless the code references IP addresses specifically. The mock server runs a DNS resolver which automatically redirects client requests for known domain names to this address ( resolver).