Merge pull request moby#9948 from tangicolin/improve-doc-networking

Improve networking documentation with default mac address range

docs/sources/articles/networking.md

@@ -12,7 +12,9 @@ private range defined by [RFC 1918](http://tools.ietf.org/html/rfc1918)
 that are not in use on the host machine, and assigns it to `docker0`.
 Docker made the choice `172.17.42.1/16` when I started it a few minutes
 ago, for example — a 16-bit netmask providing 65,534 addresses for the
-host machine and its containers.
+host machine and its containers. The MAC address is generated using the
+IP address allocated to the container to avoid ARP collisions, using a
+range from `02:42:ac:11:00:00` to `02:42:ac:11:ff:ff`.
 
 > **Note:**
 > This document discusses advanced networking configuration