Assuming that loopback interface is always alone in its network, what are the practical reasons to have IPv4 subnet prefix shorter than /32 for these interfaces?

Moreover, why according to various standards IPv4 loopback address is any address from network? Is not it a waste of about 16 million IP addresses?

https://tools.ietf.org/html/rfc3330 - 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 for loopback,
   but no addresses within this block should ever appear on any network

Interestingly, there is no such kludge for IPv6 loopback addresses where ::1/128 used as a loopback.


   The unicast address 0:0:0:0:0:0:0:1 is called the loopback address.
   It may be used by a node to send an IPv6 packet to itself.  It must
   not be assigned to any physical interface.  It is treated as having
   Link-Local scope, and may be thought of as the Link-Local unicast
   address of a virtual interface (typically called the "loopback
   interface") to an imaginary link that goes nowhere.
  • Unfortunately, this is actually off-topic as historical trivia. – Ron Maupin Nov 22 '20 at 7:39
  • Why historical? Is not it still relevant as it's still possible to choose different subnets for loopbacks? – Eugene D. Gubenkov Nov 22 '20 at 7:41
  • Only the first sentence is on-topic. The spirit of the question is the other sentences, and thus off-topic. (as I said on meta, what you do with a loopback interface is only limited by your imagination.) – Ricky Nov 24 '20 at 17:32

When the block was defined for IPv4 loopback addresses, the IPv4 addresses were classful, so you could not define a single address without the entire network class in the block.

Once software was coded to prevent use of that range in networks (it is a range that all IPv4 implementations must recognize and deal with), it became impossible to change as it would cost a lot of money to change. Also, people began using the loopback range for testing applications, sourcing from one address in the block to a different destination address in the block, and some bad programming led to some applications using the range for inter-process communication, rather than the official OS version of IPC.

  • Thanks! Does it mean that now there is really no reason to use anything but /32 for IPv4 loopback interfaces? – Eugene D. Gubenkov Nov 22 '20 at 7:43

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