a port scanner is an application designed to probe a server or host for open ports. Such an application may be used by administrators to verify the security policies of their networks and by attackers to identify network services running on a host and exploit vulnerabilities.

port-scanner.py

#!/usr/bin/env python2
from socket import * 

if __name__ == '__main__':
    target = raw_input('Enter host to scan: ')
    targetIP = gethostbyname(target)
    print 'Starting scan on host ', targetIP

    #scan reserved ports
    for i in range(20, 1025):
        s = socket(AF_INET, SOCK_STREAM)

        result = s.connect_ex((targetIP, i))

        if(result == 0) :
            print 'Port %d: OPEN' % (i,)
        s.close()

Example