Why can I ping IP addresses from 127.0.0.1 to 127.255.255.254?

Question

I thought that the loopback IP address of my machine is 127.0.0.1.

I do not understand why am I able to ping IP addresses until 127.255.255.254.

Answer 1

The entire address block 127.0.0.0/8 is the block of loopback addresses for a host. There are RFCs that explain this.

The goes back at least as far as RFC 990, ASSIGNED NUMBERS:

The class A network number 127 is assigned the "loopback" function, that is, a datagram sent by a higher level protocol to a network 127 address should loop back inside the host. No datagram "sent" to a network 127 address should ever appear on any network anywhere.

RFC 1122, Requirements for Internet Hosts -- Communication Layers:

(g) { 127, }

Internal host loopback address. Addresses of this form MUST NOT appear outside a host.

Also RFC 3330, Special-Use IPv4 Addresses:

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].

Answer 2

To answer the OP's Q: "why am I able to ping IP addresses until 127.255.255.254"? (which is a dup of https://serverfault.com/questions/360283/loopback-interface-on-linux-catches-all-loopback-traffic)

A: the early RFCs were ambiguous, Linux and apparently Windows interpreted them to mean "the host should respond on all loopback addresses. BSD - and thus macOS - use the "only 127.0.0.1/32" interpretation.

Aside: Linux's systemd relies on this behaviour - its systemd-resolved local stub listener receives requests via the loopback interface and filters out anything other than 127.0.0.53 or 127.0.0.54 (ref. https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html). At no point are the IP addresses 127.0.0.53 or 127.0.0.54 configured on lo.