[szszrk]

one thing I always forget about, is that you have a whole network of 127.0.0.0/8 , not just one IP.

So you can create multiple addresses with multiple separate "domains" mapped statically in /etc/hosts, and allow multiple apps to listen on "the same" port without conflicts.

[dietr1ch]

Unlike IPv6 localhost that's just the [::1] address. I'm not sure if you can abuse IPv4 in IPv6 to do the same

[chasd00]

I never thought of using localhost like that, I'm surprised that works actually. Typically, if you want a private /8 you would use 10.0.0.0/8 but the standard 192.168.0.0/16 gives you a lot of address space ( 255^2 - 2 IPs (iirc) ) too.

..actually this is very weird. Are you saying you can bind to 127.0.0.2:80 without adding a virtual IP to the NIC? So the concept of "localhost" is really an entire class A network? That sounds like a network stack bug to me heh.

edit: yeah my route table on osx confirms it. very strange (at least to me)

[szszrk]

That was deliberate. Works on Linux and Windows as well. I think this is the current RFC: https://datatracker.ietf.org/doc/html/rfc5735

You can do:

python3 -m http.server -b 127.0.0.1 8080

python3 -m http.server -b 127.0.0.2 8080

python3 -m http.server -b 127.0.0.3 8080

and all will be available.

Private network ranges don't really have the same purpose, they can be routed, you have to always consider conflicts and so on. But here with 127/8 you are in your own world and you don't worry about anything. You can also do tests where you need to expose more than 65k ports :)

You have to also remember these are things established likely before even DNS was a thing, IP space was considered so big that anyone could have a huge chunk of it, and it was mostly managed manually.

[dizhn]

I didn't really know the mechanism of how this worked but if you check your resolv file you might find that the nameserver IP for your localhost is 127.0.0.53 . It is so in recent Linux distros. (Probably a systemd thing)