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In the Internet Protocol Version 4, the address is a non-routable meta-address used to designate an invalid, unknown or non-applicable target. This address is assigned specific meanings in a number of contexts.

As a host address[edit]

Uses include:

  • A way to specify "any IPv4 address at all". It is used in this way when configuring servers (i.e. when binding listening sockets). This is known to TCP programmers as INADDR_ANY. (bind(2) binds to addresses, not interfaces.)
  • The address a host claims as its own when it has not yet been assigned an address. Such as when sending the initial DHCPDISCOVER packet when using DHCP.
  • The address a host assigns to itself when address request via DHCP has failed, provided the host's IP stack supports this. This usage has been replaced with the APIPA mechanism in modern operating systems.
  • A way to explicitly specify that the target is unavailable.[1]

In the context of servers, can mean "all IPv4 addresses on the local machine". If a host has two IP addresses, and, and a server running on the host is configured to listen on, it will be reachable at both of those IP addresses.

As a default route[edit]

In the context of routing, is usually used with a network mask of zero as the network address of the default route. This will often be expressed as "" or "".

This matches all address in the IPv4 address space and is present on most hosts, directed towards a local router.

In IPv6[edit]

In IPv6, the all-zeros address is typically represented by "::". [2]


  1. ^ "Cable Device Management Information Base for Data-Over-Cable Service Interface Specification (DOCSIS) Compliant Cable Modems and Cable Modem Termination Systems". If, either syslog transmission is inhibited, or the Syslog server address is not an IPv4 address. 
  2. ^ Das, Kaushik. "IPv6 Addressing". Retrieved 18 June 2015. 
  • In RFC 1122 the notation "{0,0}" is used to designate (x being anything from 0 to 32). Quote: "{ 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."

External links[edit]