Ephemeral ports


Ubuntu only uses ports >32768 for outgoing connections

/usr/src/linux/Documentation/networking/ip-sysctl.txt

 ip_local_port_range - 2 INTEGERS

         Defines the local port range that is used by TCP and UDP to
        choose the local port. The first number is the first, the 
        second the last local port number. Default value depends on
        amount of memory available on the system:
        > 128Mb 32768-61000
        < 128Mb 1024-4999 or even less.
        This number defines number of active connections, which this
        system can issue simultaneously to systems not supporting
        TCP extensions (timestamps). With tcp_tw_recycle enabled
        (i.e. by default) range 1024-4999 is enough to issue up to
        2000 connections per second to systems supporting timestamps.

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