Earlier quoted context omitted.
It's especially useful for some IPv6 addresses.
IPv6 has a different representation format, where one (and only one) sequence of fields containing only 0 bytes may be replaced by a single '::', turning '2001:0:0:1:0:2' into '2001::1:0:2'. I never quite understood why they chose to reuse the colon as a separator character, because this is problematic when you want to append a port number. '2001::1:0:2:8080' could either be '2001:0:0:1:0:2 on port 8080', or '2001:0:…
More IPv6 hackery: When using a link-local address (every interface has a link-local IPv6 address), you can append a zone index to the address in the brackets as routing information of where to send the link local request. For windows it's the interface number, for Unixes it's the interface name.
To test this out, first find a link-local address on your network. To find all link-local addresses on eth0, use this command to ping the all-nodes link-local multicast group:
ping6 -I eth0 -c 2 ff02::1
lynx https://[fe80::21e:67ff:fe08:d500%eth0]:443/