The cache contains information about errors that occur during the connection process. Some errors are considered “blocking.” If too many of these occur successively from a given host without a successful connection, the server blocks further connections from that host. The max_connect_errors system variable determines the number of permitted errors before blocking occurs.

Read more about host cache.

To unblock the host use:
mysqladmin flush-hosts
To get rid of the blocks, set in my.cnf:
skip-name-resolve