Before You Create A Network « WordPress Codex

Before You Create A Network « WordPress Codex: "You cannot create a network in the following cases:

"WordPress address (URL)" uses a port number other than ':80', ':443'.
You cannot choose Sub-domain Install (for a domain-based network) in the following cases:

The WordPress URL contains a path, not just a domain. (That is, WordPress is not installed in a document root, or you are not using the URL of that document root.)
"WordPress address (URL)" is localhost.
"WordPress address (URL)" is IP address such as 127.0.0.1."

'via Blog this'