Opera doesn't recognize 0.0.0.0 as localhost. Is this behaviour standard-conform?

Question

Opera (version 10.10) under Linux (don't know about other OSes) does not recognize http://0.0.0.0/ as local address (http://127.0.0.1/) , while Firefox does.

What is the difference between these addresses?

Which ob the two browsers in this case is standard conform?

EDIT
Not only Firefox, but also Konqueror and wget handle 0.0.0.0 same way as local address. So, could it be Linux-specific?

Answer 1

The localhost is normally 127.0.0.1. All 127.x.x.x are considered loopback. I think firefox is the exception on this case.

"Loopback" IP addresses:

    127.0.0.0 - 127.255.255.255

Each computer on the Internet uses 127.0.0.0/8 to identify itself, to itself. 127.0.0.0 to 127.255.255.255 is earmarked for what is called "loopback". This construct allows a computer to establish/validate its IP stack. Most software only uses 127.0.0.1 for loopback purposes (the other addresses in this range are seldom used). All of the addresses within the loopback address are treated with the same levels of restriction in Internet routing, so it is difficult to use any other addresses within this block for anything other than node specific applications, generally bootstraping. This is documented in RFC 3330.

(taken from IANA.org)

The 0.0.0.0 address is:

0.0.0.0/8 reserved for self-identification [RFC5735]

(taken again from IANA)

and in details:

0.0.0.0/8 - Addresses in this block refer to source hosts on "this" network. Address 0.0.0.0/32 may be used as a source address for this host on this network; other addresses within 0.0.0.0/8 may be used to refer to specified hosts on this network ([RFC1122], Section 3.2.1.3).

(taken from RFC 5735)

Answer 2

According to Wikipedia, the only correct address (reserved explicitly for that purpose) is 127.0.0.1. I don't think it's correct to treat 0.0.0.0 as localhost - Firefox doesn't do it for me on Windows 7, anyway.

Answer 3

This link says that 0.0.0.0 is used for "broadcast messages to the current ("this") network as specified by RFC 1700, page 4."

I would suggest that Opera is correct, as Firefox is interpreting an address that is only valid as a source address as a destination address.