URL-SYNTAX: add default port numbers and IDNA details
Closes #6316
diff --git a/docs/URL-SYNTAX.md b/docs/URL-SYNTAX.md
index f724f75..d8fcc92 100644
--- a/docs/URL-SYNTAX.md
+++ b/docs/URL-SYNTAX.md
@@ -167,15 +167,30 @@
http://[2001:1890:1112:1::20]/
+### IDNA
+
If curl was built with International Domain Name (IDN) support, it can also
handle host names using non-ASCII characters.
+curl supports IDN host names using the IDNA 2008 standard. This differs from
+browsers that follow the WHATWG URL spec, which dictates IDNA 2003 to be used.
+The two standards have a huge overlap but differ slightly, perhaps most
+famously in how they deal with the German "double s" (`ß`).
+
## Port number
If there's a colon after the hostname, that should be followed by the port
number to use. 1 - 65535. curl also supports a blank port number field - but
only if the URL starts with a scheme.
+If the port number is not specified in the URL, curl will used a default port
+based on the provide scheme:
+
+DICT 2628, FTP 21, FTPS 990, GOPHER 70, HTTP 80, HTTPS 443, IMAP 132,
+IMAPS 993, LDAP 369, LDAPS 636, MQTT 1883, POP3 110, POP3S 995, RTMP 1935,
+RTMPS 443, RTMPT 80, RTSP 554, SCP 22, SFTP 22, SMB 445, SMBS 445, SMTP 25,
+SMTPS 465, TELNET 23, TFTP 69
+
# Scheme specific behaviors
## FTP
