Scanners are important for the security of networks because they are essential 
tools for those who want to attack a system. 

The preparation of an attack by an intruder could look as follows:
- Scan a target machine or selected network
- Observe which services are offered and which operating systems runs these 
  services, and work on some well-known vulnerability in any of them 
- Scan any network or machine, look for a service or operating system (including 
  the checkup of the version) with a known vulnerability.
- Execute expliot code or guess system password, and assume system and/or network control
- Deploy backdoor and clean intruder's tracks
- Access network and/or system at will using backdoor, even if original hole
  is closed by sysadmin.
- Use compromised network as a springboard for further attacks on other networks.
- An intruder may also target a user's E-mail system, physical security, or broadband 
  connection, deposit a trojan horse, worm, or other backdoor and by-pass internal 
  and/or external security controls through a connected corporate VPN, from a 
  compromised internal machine, and/or through a dial-in