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?

Answer 1

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.

Answer 2

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.
• http://tools.ietf.org/html/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.
• http://tools.ietf.org/html/rfc1122#section-3.2.1.3, which describes the specific usage of the 0/8 network.

Answer 3

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].

Answer 4

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.