RHN Classic にシステムを登録している時、"We could not contact Red Hat Network (xmlrpc.rhn.redhat.com)" のエラーが発生します

Issue

• RHN Classic にシステムを登録している時、次のエラーが発生します。

Error message:
    We could not contact Red Hat Network (xmlrpc.rhn.redhat.com)
    Make sure the network connection on this system is operational.

    Did you mean to register to a Red Hat Network Satellite or Proxy?If so, you can enter Satellite or Proxy
    location instead.

    You will not be able to successfully register this system without contacting a Red Hat Network server.