What does column 'tcp6' mean on output netstat? Please anyone explain the follow output of netstat:

tcp6 0 0 dmz.local.net:www ESTABLISHED 21393/apache2

What does tcp6 mean?

  • 1
    tcp6 refers to the TCP/IP version 6 (IPv6) protocol that your apache is using to connect to the external host. Just tcp would mean that the TCP/IP version 4 (IPv4) that is being used
    – debal
    Mar 20 '14 at 8:49
  • 6
    tcp6 supports both IPv6 and IPv4 protocols
    – debal
    Mar 20 '14 at 8:55
  • Interesting is that netstat -nN* shows ip4 address at tcp6 line. tcp6 0 0
    – TMa
    May 18 '16 at 20:29

tcp6 simply means TCP protocol over IP v6.

tcp6 0 0 dmz.local.net:www ESTABLISHED 21393/apache2

As from the netstat manual:

  • tcp6: The protocol used. Here it is TCP over IPv6

  • 0: The count of bytes not copied by the user program connected to this socket.

  • 0: The count of bytes not acknowledged by the remote host. Local Address

  • dmz.local.net:www: Address and port number of the local end of the socket. Unless the (-n) option is specified, the socket address is resolved to its canonical host name (FQDN), and the port number is translated into the corresponding service name.

  • Address and port number of the remote end of the socket.

  • ESTABLISHED: The state of the socket. The state ESTABLISHED means the socket has an established connection.

  • 21393/apache2: Slash-separated pair of the process id (PID) and process name of the process that owns the socket.

To sum up: your local apache2 process (pid= 21393), listening on the standard www port (80) has established a TCP (over IPv6) connection with the remote host on port 49964 (unresolved IPv6 address which is a link-local IPv6 address: an address that a computer assigns itself in order to facilitate local communications).

For more about IPv6:


This means that a connection using TCP/IP version 6 protocol was made and established toward your Apache web server.

TCP/IP version 6 is the follower of TCP/IP version 4. One of the main difference between the two is the number of IP addresses you can use in IPv6 is far higher of the one allowed by IPv4.

You see also the name and the port at each side of the connection. dmz.local.net on port 80 (www) beeing your side, the daemon apache2 running as PID 21393. The connection is ESTABLISHED, meaning that the TCP handshake is successfully accomplished and the client is ready to send information to the server and the server ready to reply the requests from the client.


It means your apache2 service is using IPv6.

apache2 will listen on port 80 of your server's IP address.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.