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I'm having trouble differentiating the two on which is the unusable address. I've found that IP 127.0.0.0 is reserved for loopback, and 0.0.0.0 is valid but have not found a clear answer.

Thanks in advance

Caiz
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1 Answers1

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In the context of servers, 0.0.0.0 means "all IPv4 addresses on the local machine". If a host has two IP addresses, 192.168.1.1 and 10.1.2.1, and a server running on the host listens on 0.0.0.0, it will be reachable at both of those IPs.

In the context of routing, 0.0.0.0 usually means the default route, i.e. the route which leads to "the rest of" the internet instead of somewhere on the local network.

Soumen Mukherjee
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  • In terms of routing you always need a prefix length. Routing uses only prefixes, not separate addresses. So `0.0.0.0/0` is indeed the default route (prefix length 0, so everything matches), but `0.0.0.0/1` is also a valid route for the first half of the address space (prefix length 1, so the first bit has to be 0, excludes everything where the first bit is 1), etc. – Sander Steffann Oct 01 '17 at 10:35