Is 0.1.2.3 a valid IP address?

Question

Are IP addresses with all zeroes in the first octet valid?

For example, can 0.1.2.0/24 be a valid subnet, with network address 0.1.2.0, broadcast address 0.1.2.255 and an usable address range from 0.1.2.1 to 0.1.2.254?

It looks like it should be valid, but it doesn't work, at least on Windows systems.

If it's not valid, then why?

score 36 · Accepted Answer · edited Oct 07 '21 at 06:47

36

RFC1122, Requirements for Internet Hosts -- Communication Layers, says:

     { <Network-number>, <Host-number> }

(a)  { 0, 0 }

     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.

     See also Section 3.3.6 for a non-standard use of {0,0}.

(b)  { 0, <Host-number> }

     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.

edited Oct 07 '21 at 06:47

Community

1

answered Aug 11 '11 at 20:30

wfaulk

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OK, not valid. Is any IP, other than 0.0.0.0 in DHCP, actually used? – Mark Wagner Aug 11 '11 at 22:06
To my knowledge, no. – wfaulk Aug 11 '11 at 22:41
4

It would appear that the intention was that a host might know its own host number, but not its network number, and use this to request that information from a DHCP-like server. That said, I'm not aware that anything like this ever existed. It also probably doesn't make a lot of sense in a post-classful world. – wfaulk Aug 12 '11 at 16:50
Wow, does it mean, that if I am on a local network, like on 10.5.0.0/16, then I can access the other machines with the IP 10.5.x.y on the same network on the IP 0.0.x.y ? – peterh Nov 15 '20 at 13:54

score 19 · Answer 2 · edited Oct 07 '21 at 06:47

Looks like 0.0.0.0/8 is in the list of IANA Reserved subnets.

http://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xml, the list of IANA reserved addresses, since this one dates far enough back (1981!) to have been an IANA reserved address range.
https://www.rfc-editor.org/rfc/rfc5735, describing special uses of IPv4, of which this subnet is a member. The most-current list of special subnets, including the subnets reserved for documentation purposes.
https://www.rfc-editor.org/rfc/rfc1122#section-3.2.1.3, which describes the specific usage of the 0/8 network.

score 13 · Answer 3 · edited Oct 07 '21 at 06:47

13

Per RFC 5735, 0.0.0.0/8 is a reserved IP address range, as follows:

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 [RFC1700, page 4].

edited Oct 07 '21 at 06:47

Community

1

answered Aug 11 '11 at 20:34

Skyhawk

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score 11 · Answer 4 · edited Oct 07 '21 at 06:47

11

According to RFC 1700 - "Assigned Numbers", "Special Addresses" section, a network number of 0 can only be used as a source address, and represents a host on the same network. Therefore it is invalid to assign an address of this type to an interface.

edited Oct 07 '21 at 06:47

Community

1

answered Aug 11 '11 at 20:30

Ignacio Vazquez-Abrams

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score 0 · Answer 5 · answered Sep 12 '17 at 13:00

0

"0." networks are used for SNM ( SYSTEM NETWORK MANAGEMENT) and can't be used for something else

answered Sep 12 '17 at 13:00

Cassie Mod

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Do you have any citations to back this up? The other answers here includes quotes and links to authorities. – chicks Sep 12 '17 at 15:57

score -1 · Answer 6 · edited Dec 04 '14 at 08:54

For anyone wondering why the Network-number is 0 in this case, 0.1.2.0/24 breaks down as follows:

Network = (8 bits) 0
Subnetwork (16 bits) = 1.2
Host part = remaining 8 bits

Given that the MSB of the first octet is 0, it must be class A, with an implied network portion of 8 bits. It is subnetted a further 16 bits (the next two octets).

Given things like CIDR and the death of classful addressing, 0.1.2.0/24 should be a valid subnet (there are bits set in the network portion if you consider the first 24 bits).

score -1 · Answer 7 · answered Apr 07 '19 at 08:32

-1

The IP 0.0.0.0 is used just for request a valid IP Address from DHCP Server, and DHCP server is replying a dynamic valid IP for request computer.

answered Apr 07 '19 at 08:32

Zabihullah Ibrahimi

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Is 0.1.2.3 a valid IP address?

7 Answers7