notes about using sharding with "localhost"

Author: Dan Crosta

source/administration/sharding.txt

@@ -183,6 +183,19 @@ scratch, use the following procedure as a starting point:
    If you do not specify a shard key, MongoDB will shard the
    collection using the ``_id`` field, and this is rarely ideal.
 
+Testing Sharding with "localhost"
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Because all components of a sharded cluster must be able to reach one
+another over the network, there are special restrictions to be aware of
+when configuring sharding on a single machine.
+
+If you use either "localhost" or "127.0.0.1" as the hostname portion of
+any host identifier (the ``host`` argument to ``addShard``, or the value
+to the ``--configdb`` setting for :program:`mongos`), then you must use
+"localhost" or "127.0.0.1" for all such settings. If you do not, you
+will receive an error.
+
 Cluster Management
 ------------------