Add Determine ipv4 And ipv6 Public IP Addresses as a Unix TIL

unix/determine-ipv4-and-ipv6-public-ip-addresses.md

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+# Determine ipv4 And ipv6 Public IP Addresses
+
+There are a number of ways to do this. The one that I've settled on is sending a
+`curl` request to a public URL that was specifically set up to echo back the
+public IP of the device making the request. There are many such URLs, but the
+one that I tend to use is `ifconfig.io`.
+
+When I run this as is, I get something like the following which you may
+recognize as an _ipv6_ IP address.
+
+```bash
+$ curl ifconfig.io
+2001:db8:3333:4444:5555:6666:7777:8888
+```
+
+This is because if ipv6 is available, like it is for me, `curl` is going to
+prefer that.
+
+Now, if I'm trying to track down specifically my ipv4 address, I can use the
+`-4` flag (or `--ipv4`).
+
+```bash
+$ curl -4 ifconfig.io
+73.23.45.157
+```
+
+Similarly, I could explicitly specify ipv6 with `-6` or `--ipv6`.
+
+See `man curl` for more details.