When used as a TCP destination address (on Linux at least), 0.0.0.0 is equivalent to 127.0.0.1, and :: is equivalent to ::1. The resulting connections will eve

When used as a TCP destination address (on Linux at least), 0.0.0.0 is equivalent to 127.0.0.1, and :: is equivalent to ::1. The resulting connections will even pick 127.0.0.1/::1 as their source address, or fail to complete if no IPv4/IPv6 loopback address is configured. There's also ::ffff:0.0.0.0 and ::ffff:127.0.0.1, which behave like their corresponding IPv4 addresses if IPV6_V6ONLY isn't enabled on the socket. In any case, if you have server code that only listens on 0.0.0.0 (or ::ffff:0.0.0.0, if you're crazy), it's obsolete now because it's unable to accept IPv6 connections. A modern server should either listen on :: with IPV6_V6ONLY=0, or set IPV6_V6ONLY=1 and create two sockets listening on :: and 0.0.0.0. One annoying thing about the socket API is that binding to loopback always requires two sockets, one for 127.0.0.1 (or ::ffff:127.0.0.1) and another for ::1. And if you ever create a listen API that accepts "localhost:8080" as input, but only creates one socket for one address, then a pox on your house.