This security approach is based on the concept of building several layers of defense around the host or information resource that you want to protect against malicious attacks.
In enterprise IT networks, the defense in depth approach is implemented using different security infrastructure equipment and mechanisms. These include hardware firewalls, Intrusion Prevention Systems, Authentication and Authorization servers, encryption devices, proxy servers, enterprise antivirus software, host intrusion detection software etc. Depending how big the enterprise is and how much security level they want to achieve, security administrators use some or all of the mentioned security tools.
On the other hand, for small home networks to attain a strong defense in depth I would suggest at least the following:
- Hardware Firewall (usually built in the internet broadband router)
- Software Firewall on the internal PCs
- Antivirus/Antispyware security suit
The hardware firewall will secure the internal home network from attacks initiated directly from the internet. The software firewall (with both inbound and outbound connection control) will protect the PC itself from malicious scripts or Trojan horse software that have managed to penetrate the external perimeter via connection initiated from the inside (e.g through the browser session, through email attachments etc). The outbound connection control of the software firewall will alert the user if a malicious script or malware tries to open connection from the PC itself. Last but not least, a complete antivirus/antispyware security package will protect our computer from viruses, worms, spyware tools, keylogers etc.
Do not forget also to update operating systems regularly.