How to find the number of open ports in linux?

How to find the number of open ports in linux? I want to see if I am running out of ports. Also, how do I see the limit of my OS?

From serverfault erotsppa

• netstat will allow you to see what ports are open, do "netstat -" to see what fits your needs best.

  Paul Tomblin : `netstat --inet` will help the most.

  jer.salamon : I meant -? missing character.

  MarkM : or read the manpage

  Tobu : Also include --inet6 (short for both: -4 -6), to get IPv6 sockets and ip-agnostic sockets (the latter being the default on dual stack hosts, see rfc 3493 section 3.7).
  From jer.salamon

• As others have mentioned, netstat is the tool to use to determine what ports are in use currently. As to the limits, the number of ports available are a 16bit unsigned integer which gives you the range 0-65535. The ports that are available for applications to bind to are the reserved privileged/root ports (0-1024) plus whatever is not covered by your ephemeral port range.

  You can view your ephemeral ports by running cat /proc/sys/net/ipv4/ip_local_port_range.

  To modify that persistently, you would have to add/modify "net.ipv4.ip_local_port_range" in the /etc/sysctl.conf file, or interactively with sysctl -n net.ipv4.ip_local_port_range="<start_port> <end_port>"

  Joel K : nit picking, but it's not exactly a ipv4 limit. It's a tcp/udp limit. and those run independently of ipv4. (ex. ipv6 doesn't do anything for transport layer)

  Alex : Aaah, you are right. I have removed the IPV4 reference in my answer.
  From Alex

• Personally I prefer nmap. You can find the state of all ports by issuing nmap -P 1-65535 target. Most distributions should have this package available via their package manager.

  From ThaKidd

• 'nmap localhost' will give you all your open ports and services running on them.

  From Vivek Varghese Cherian

• netstat -a46 | grep ESTABLISHED | wc -l
  

  compared to

  cat /proc/sys/net/ipv4/ip_local_port_range
  

  erotsppa : the -a46 didn't work. Any help?

  Grizly : what distro you running? (that works on ubuntu server 10.04 LTS). Of course, if you don't have ipv6 installed, then just use netstat -a.

  Grizly : Tested on my CentOS box, seems it hangs if you don't use "-n" to stop name resolution. (netstat -an | grep ESTABLISHED | wc -l)
  From Grizly

• netstat -tulnp

  The arguments to the netstat program are listed below:

  *
  
    t - Show TCP
  *
  
    u - Show UDP
  *
  
    l - Show only listening processes (netstat can show both listening and all established connections, i.e. as a client too)
  *
  
    n - Do not resolve network IP address names or port numbers
  *
  
    p - Show the process name that is listening on the port
  

  From Rajat