I read that this pool:

  • Means "these hosts" in "this network"
  • Is used for self-identification
  • Is used for broadcast messages

My questions are:

  • Where exactly these addresses can be found?
  • Which protocols use them?

According to RFC 5735: - Addresses in this block refer to source hosts on "this" network. Address may be used as a source address for this host on this network; other addresses within may be used to refer to specified hosts on this network ([RFC1122], Section

So, looking at RFC 1122:

This host on this network. MUST NOT be sent, except as a source address as part of an initialization procedure by which the host learns its own IP address. (i.e. any IP from to

Specified host on this network. It MUST NOT be sent, except as a source address as part of an initialization procedure by which the host learns its full IP address.

So @Spiff is right, they're used exclusively in initialization procedures before obtaining the "real" IP Address.

But later on RFC 1122 states that:

There is a class of hosts (4.2BSD Unix and its derivatives, but not 4.3BSD) that use non-standard broadcast address forms, substituting 0 for 255 (as in instead of

so BSDs version 4.2 actually send broadcasts to


Found another piece of info: is INADDR_ANY [snip] Another common use for is to designate ifIndex numbers, as for the "Link Data" value in OSPF, and for some BSD-ish interfaces.

And here appears BSD again:

imr_interface should be set to INADDR_ANY to choose the default multicast interface, or the IP address of a particular multicast-capable interface if the host is multihomed. Since FreeBSD 4.4, if the imr_interface mem- ber is within the network range, it is treated as an interface index in the system interface MIB, as per the RIP Version 2 MIB Extension (RFC-1724).

  • To summarize: if only one host at a time needs an IP address, then is used. If MANY hosts at a time need ip addresses, then is used to DISTINCT them? Feb 10 '12 at 7:54
  • @rz00 that's the way I understand it.
    – Mr Shunz
    Feb 10 '12 at 8:19
  • 2
    @rzr00: If many hosts at the same time are requesting IP addresses, they usually still use 0/32 fine -- they can still be distinguished by their link-level addresses (e.g. MAC addresses on Ethernet).
    – user1686
    Feb 10 '12 at 15:20
  • @grawity but how the hosts getting the requested IP address if it is forbidden to use 0/32 as a destination, only as a source, hence a DHCP server can't do anything with the request it got from such an address.
    – Hi-Angel
    Sep 4 '14 at 15:19
  • @grawity oh, never mind, I got it. The quote about an exception is except as a source address as part of an initialization procedure by which the host learns its full IP address, so for the time of DHCP request the DHCP server can send an answer to this address just fine.
    – Hi-Angel
    Sep 4 '14 at 15:24

The all-zeroes address is used by clients that don't know their own address yet, such as BootP/DHCP clients that have just booted or just connected to a new network and haven't been assigned an IP address from the BootP/DHCP server yet.

I look at a lot of Ethernet LAN traffic while debugging problems, and that's the only place I can think of that I've ever seen in use.

  • For computer that doesn't know its address, is used. It is only address. I'm talking about network where addresses from to could be used. Feb 9 '12 at 20:35

Where exactly these addresses can be found?

On networked machines that are currently negotiating for actual IP addresses.

Which protocols use them?

These addresses are used temporarily during ARP probes:

An ARP probe is an ARP request constructed with an all-zero sender IP address. The term is used in the IPv4 Address Conflict Detection specification (RFC 5227). Before beginning to use an IPv4 address (whether received from manual configuration, DHCP, or some other means), a host implementing this specification must test to see if the address is already in use, by broadcasting ARP probe packets.


<guess type="wild" reliability="low">

Before CIDR, IP addresses were separated into a "network" part and "host" part, for example, would have as a class-A network and as the host number. It's possible that 0/8 was intended to allow for easy reassignment of a network number while keeping the host numbers in it static.

For example, a host could use as the "source" address for some-or-other protocol, asking "I know I am host 2.3.4 in a class-A network, but I need someone to tell me my network number". These days, this is never done, since the length of the host part can vary a lot, and it's highly unlikely that the same host number will be available for use in the new network.


  • 1
    It makes sense, but it's still a guess. I'm asking about this network, because information about it are unclear. Everywhere they're writing that it's "reserved" or for "auto-identification" purpose, and I didn't see clear example where this pool is used. Feb 10 '12 at 16:29

Looks like it's reserved by IANA - the organisation that hands out IP addresses see 000/8 in the table. The table shows 000/8 to 255/8 that's to

I think in Windows if ipconfig shows it, it means no ip address assigned, like ipconfig /release might or would cause it.

and in netstat -n whether it's listed as Listening or a private address from RFC 1918 like or if it's 127/8 they tell you who can connect, so if then anybody, if then only them, and if 127/8 then only local host.

As to what protocol, all I can say is IP since they're IP Addresses. But maybe some higher layer protocols do.. It's not for broadcasts. And it wouldn't identify any more than any othe ip adress or network, infact less since a computer on a network shouldn't have that IP address.

In windows when a computer has that IP it is not on a network. (correction, it would be on a network if there's a wired or wireless connection, and as grawity points out, they'd still have their MAC addresses)


Prefix Designation Date Whois Status Note
000/8 IANA - Local Identification 1981-09 RESERVED
001/8 APNIC 2010-01 whois.apnic.net ALLOCATED
002/8 RIPE NCC 2009-09 whois.ripe.net ALLOCATED
003/8 General Electric Company 1994-05 LEGACY
004/8 Level 3 Communications, Inc. 1992-12 LEGACY
005/8 RIPE NCC 2010-11 whois.ripe.net ALLOCATED

  • As I've mentioned in comment above, I'm talking about addresses from to They aren't in ipconfig output. Feb 9 '12 at 20:35

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