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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.

  • 2
    I think your question is off-topic here, but I would like to tell you that the entire range 127.0.0.0/8 (127.0.0.1 - 127.255.255.254) is reserved for loopback purposes and they are always available. On most systems, localhost resolves to 127.0.0.1 which is the most commonly used IPv4 loopback address. – Hung Tran Oct 2 '17 at 8:08
  • @HungTran Thank you for the information. So basically my own computer has many local IP addresses? – yoyo_fun Oct 2 '17 at 8:45
  • Yes, and the right term should be loopback IP addresses, NOT local. – Hung Tran Oct 2 '17 at 9:11
  • @HungTran Thank you, I understand. So the computer can have an IP address on the NIC and that is the local IP address, the IP assigned to the NIC. The loopback is a separate IP address but it is not assigned to the NIC it is just an internal IP address that the computer uses to communicate with itself. Am I correct? – yoyo_fun Oct 2 '17 at 9:14
  • Basically, we can understand like that. You can read more discussions here. – Hung Tran Oct 2 '17 at 9:36
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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].

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