根据rfc3330，环回块是 127.0.0.0/8：

   127.0.0.0/8 - This block is assigned for use as the Internet host
   loopback address.  A datagram sent by a higher level protocol to an
   address anywhere within this block should loop back inside the host.
   This is ordinarily implemented using only 127.0.0.1/32 for loopback,
   but no addresses within this block should ever appear on any network
   anywhere [RFC1700, page 5].

这个块这么大是有原因的吗？有没有配置127.0.0.0/8以外的127.0.0.1/8以外的其他地址的场合？