Reserved (IANA)
0.0.0.0 has different meanings depending on context. As a source address (typically during DHCP discovery, before a host has obtained an IP), it means "I don’t know my own address yet". As a bind address for a server (`bind 0.0.0.0:8080`), it means "listen on every network interface this machine has". As a routing destination, it represents the default route — "anywhere not otherwise specified".
Backend developers see 0.0.0.0 constantly. Servers bind to 0.0.0.0 in Docker containers so the service is reachable from any network the container is attached to. Static routing tables list `0.0.0.0/0` as the default route to send unmatched traffic to the gateway. Some malware blocklists redirect blocked domains to 0.0.0.0 so connections fail silently rather than hitting a real server.
Defined in RFC 1122. The full 0.0.0.0/8 block is reserved for "source addresses". 0.0.0.0/0 in routing notation matches every possible IPv4 address — the universal default route. The IPv6 equivalent is `::` (just two colons).
Because the resolver thinks example.com lives at 0.0.0.0, but 0.0.0.0 is not routable — connections fail immediately. This is faster and less noisy than 127.0.0.1, which would actually try to connect to your own machine.
It can be — the server becomes reachable on every interface, including the public one if the machine has a public IP. Bind to 127.0.0.1 if you only want local access. Use 0.0.0.0 inside containers where the host firewall controls exposure.
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