It seems that the command line is transposing the host and db when using the host formats specified in the command line usage text:

192.168.0.5/foo

or

192.168.0.5:9999/foo

I receive the following errors respectively when connecting using these formats to a valid host and DB.

Error: Database name cannot have reserved characters for mongodb:// URL: mongodb://foo:27017/192.168.0.5

Error: Database name cannot have reserved characters for mongodb:// URL: mongodb://foo:27017/192.168.0.5%3A9999

If I provide an incorrect host specification as

mongo -u <user> -p <pass> foo/192.168.0.5:9999