size of IPv4 loopback address block According to rfc3330 the loopback block is 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]. Is there a reason why this block is so huge? Are there occasions where any other addresses from 127.0.0.0/8 besides 127.0.0.1/8 are configured?

It comes from the times of clasful IP and 127.0.0.0 is an class-A network and therefore a /8 mask is assigned.

To answer the second part: Yes I've seen setups where more than 127.0.0.1 was used, e.g. for running multiple SMTP content filters on one host or multiple instances of a database server.

Note that for IPv6 you only have one loopback address ::1