ruby - ruby で net/ftp を使用して接続できない

Question

FTP サーバーに接続しようとすると、次のエラーが発生します。

ftp = Net::FTP::new("example.sg","username","password")
Errno::ECONNREFUSED: Connection refused - Connection refused
   from org/jruby/ext/socket/RubyTCPSocket.java:121:in `initialize'
    from org/jruby/RubyIO.java:876:in `new'
    from org/jruby/ext/socket/RubyTCPSocket.java:147:in `open'
    from /opt/jruby/lib/ruby/1.8/net/ftp.rb:159:in `open_socket'
    from /opt/jruby/lib/ruby/1.8/net/ftp.rb:175:in `connect'
    from /opt/jruby/lib/ruby/1.8/monitor.rb:191:in `mon_synchronize'
    from /opt/jruby/lib/ruby/1.8/net/ftp.rb:174:in `connect'
    from /opt/jruby/lib/ruby/1.8/net/ftp.rb:136:in `initialize'
    from (irb):7:in `evaluate'
    from org/jruby/RubyKernel.java:1083:in `eval'
    from /opt/jruby/lib/ruby/1.8/irb.rb:158:in `eval_input'
    from /opt/jruby/lib/ruby/1.8/irb.rb:271:in `signal_status'
    from /opt/jruby/lib/ruby/1.8/irb.rb:155:in `eval_input'
    from org/jruby/RubyKernel.java:1410:in `loop'
    from org/jruby/RubyKernel.java:1183:in `catch'
    from /opt/jruby/lib/ruby/1.8/irb.rb:154:in `eval_input'
    from /opt/jruby/lib/ruby/1.8/irb.rb:71:in `start'
    from org/jruby/RubyKernel.java:1183:in `catch'
    from /opt/jruby/lib/ruby/1.8/irb.rb:70:in `start'
    from /opt/jruby/bin/jirb:13:in `(root)

ftp://example.sgを使用してブラウザーから同じ Web サイトにアクセスできます。net/ftp でサイトへのアクセスに問題が発生する理由はありますか?