Syntax:
SHOW SLAVE HOSTS

Displays a list of replication slaves currently registered with the
master. Only slaves started with the --report-host=host_name option are
visible in this list.

The list is displayed on any server (not just the master server). The
output looks like this:

mysql> SHOW SLAVE HOSTS;
+------------+-----------+------+-----------+
| Server_id  | Host      | Port | Master_id |
+------------+-----------+------+-----------+
|  192168010 | iconnect2 | 3306 | 192168011 |
| 1921680101 | athena    | 3306 | 192168011 |
+------------+-----------+------+-----------+

o Server_id: The unique server ID of the slave server, as configured in
  the server's option file, or on the command line with
  --server-id=value.

o Host: The host name of the slave server, as configured in the
  server's option file, or on the command line with
  --report-host=host_name. Note that this can differ from the machine
  name as configured in the operating system.

o Port: The port the slave server is listening on.

o Master_id: The unique server ID of the master server that the slave
  server is replicating from.

Some MySQL versions report another variable, Rpl_recovery_rank. This
variable was never used, and was eventually removed. (Bug #13963)

URL: http://dev.mysql.com/doc/refman/5.1/en/show-slave-hosts.html

