Reserved (IANA)
The entire 127.0.0.0/8 block is reserved by IANA as the loopback range, with 127.0.0.1 being its most-used member. When a program connects to 127.0.0.1, the operating system’s networking stack short-circuits the packet so it never touches a physical network — it loops directly back to a process listening on localhost.
Developers run local web servers on 127.0.0.1:3000, 127.0.0.1:8080, etc. Databases like PostgreSQL and MySQL bind to 127.0.0.1 by default so only local processes can connect. /etc/hosts entries pointing custom domains to 127.0.0.1 enable local development of multi-host applications.
Defined in RFC 1122 and RFC 5735. The entire 127.0.0.0/8 block is reserved — that is over 16 million addresses, all equivalent to localhost, though almost no OS actually uses anything other than 127.0.0.1. The IPv6 equivalent is ::1 (a single address rather than a /8 block).
No. Packets destined for 127.0.0.1 are never routed onto a network — they only exist within your own machine.
It tests that the local TCP/IP stack itself is functioning. A failure indicates a broken networking driver or OS configuration.
127.0.0.1 is IPv4 loopback; ::1 is the IPv6 equivalent. Both refer to "this machine".
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