Try this out: you can access your SvelteKit app locally at http://[::1]:5173…crazy looking address, huh?

The following is information specific to a SvelteKit app running on macOS but should apply to any app that runs Node.js 17 or up for a server.

When running a SvelteKit app locally, it gives your app the default address of localhost:5173. Normally, this would mean you could access the site at as well, given that the loopback IP is normally mapped to localhost. On macOS, if you cat /etc/hosts, you’ll see something like:

# Host Database
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##	localhost	broadcasthost
::1             localhost

This shows that is mapped to localhost. So what’s going? In Node.js 17, there was a change to the order in which DNS lookup results were ordered so that instead of re-ordering results to put IPv4 first, it now uses the OS’s results as-is. This means a Node.js 17+ server is binding to the IPv6 address instead of the IPv4, so you can access your site at the “weird” looking address of http://[::1]:5173.