What is TCP6 773/cupsd ? why is it listening and why the ip is not cleared?

Question

Hello everybody, Testing my open ports i ran command sudo netstat -ntlup with a normal looking output on my ubuntu 12.04 desktop.

Active Internet connections (only servers)

Proto Recv-Q Send-Q Local Address           Foreign Address         State      PID/Program name

tcp        0      0 127.0.0.1:7634          0.0.0.0:*               LISTEN      1919/hddtemp    
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN      9393/dnsmasq    
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      773/cupsd       
tcp6       0      0 ::1:631                 :::*                    LISTEN      773/cupsd       
udp        0      0 127.0.0.1:53            0.0.0.0:*                           9393/dnsmasq

But I can't see the port TCP6 773 is listening at i.e.

tcp6 0 0 ::1:631 :::* LISTEN 773/cupsd

What is this?

Answer 1

The tcp6 means an IPv6 address is being used, as evidenced by the ::1:631 :::* listing. Also, it's a localhost connection as ::1 is the IPv6 address used for the lo network interface, similar to 127.0.0.1 on IPv4.

All of the addresses listed in your question, are only accessible via the lo interface, and not from the external Internet.