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

README.md

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 - [Curl With Cookies](unix/curl-with-cookies.md)
 - [Curling For Headers](unix/curling-for-headers.md)
 - [Curling With Basic Auth Credentials](unix/curling-with-basic-auth-credentials.md)
+- [Determine ipv4 And ipv6 Public IP Addresses](unix/determine-ipv4-and-ipv6-public-ip-addresses.md)
 - [Different Ways To Generate A v4 UUID](unix/different-ways-to-generate-a-v4-uuid.md)
 - [Display All The Terminal Colors](unix/display-all-the-terminal-colors.md)
 - [Display Free Disk Space](unix/display-free-disk-space.md)

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

@@ -0,0 +1,29 @@
+# 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.