What does it mean to bind() a socket to any address other than localhost?
I don't understand what it means to bind a socket to any address other than 127.0.0.1 (or ::1, etc.).
Am I not -- by definition -- binding the socket to a port on my own machine.. which is localhost?
What sense does it make to bind or listen to another machine or IP address's port?
Conceptually, it just doesn't make sense to me!
(This has proven surprisingly hard to Google... possibly because I'm not Googling the right terms.)
Answer
Binding of a socket is done to address and port in order to receive data on this socket (most cases) or to use this address/port as the source of the data when sending data (for example used with data connections in FTP server).
Usually there are several interfaces on a specific machine, i.e. the pseudo-interface loopback where the machine can reach itself, ethernet, WLAN, VPN... . Each of these interfaces can have multiple IP addresses assigned. For example, loopback usually has 127.0.0.1 and with IPv6 also ::1, but you can assign others too. Ethernet or WLAN have the IP addresses on the local network, i.e. 172.16.0.34 or whatever.
If you bind a socket for receiving data to a specific address you can only receive data sent to this specific IP address. For example, if you bind to 127.0.0.1 you will be able to receive data from your own system but not from some other system on the local network, because they cannot send data to your 127.0.0.1: for one any data to 127.0.0.1 will be sent to their own 127.0.0.1 and second your 127.0.0.1 is an address on your internal loopback interface which is not reachable from outside.
You can also bind a socket to a catch-all address like 0.0.0.0 (Ipv4) and :: (Ipv6). In this case it is not bound to a specific IP address but will be able to receive data send to any IP address of the machine.