Cyberstudy
PDF p.253 In progress Not in the PDF's table of contents

Screened Subnet (Perimeter Network)

Open PDF at p.253 4 flashcards

Summary

PDF p.253

A screened subnet creates a neutral zone between an internal network and the Internet, hosting public-facing servers while protecting sensitive internal resources.

In plain words

Supplementary — not from your PDF

A screened subnet (formerly called a DMZ) is a middle zone for public-facing services such as web, email, DNS and FTP. One firewall between the Internet and the screened subnet allows public traffic in to those services. A second firewall between the screened subnet and the internal network blocks almost everything, so a compromised public server can't easily reach internal systems.

Detailed explanation

PDF p.253
  • Purpose: Reduces exposure of internal network resources to external threats.
  • Typical Services: Web, email, DNS, FTP.

Firewall Configuration

  • First Firewall: Between the Internet and the screened subnet, allows traffic to public services.
  • Second Firewall: Between the screened subnet and the internal network, blocks most traffic from the screened subnet.

Important terms

taken from the text above
Typical Services
Web, email, DNS, FTP.
First Firewall
Between the Internet and the screened subnet, allows traffic to public services.
Second Firewall
Between the screened subnet and the internal network, blocks most traffic from the screened subnet.

Examples & real-world scenarios

Supplementary — not from your PDF
  • A web server in the screened subnet, with its database kept on the internal network.
  • An email gateway in the screened subnet relaying mail inward.
  • Public DNS hosted in the screened subnet.

Scenario

A company hosts its website on a server inside the internal LAN. Moving it to a screened subnet between two firewalls means that if it's compromised, the attacker still faces another firewall before reaching internal data.

Common mistakes

Supplementary — not from your PDF
  • Putting sensitive data stores in the screened subnet.
  • Allowing broad traffic from the screened subnet into the LAN.

Practical skills

Supplementary — not from your PDF
  • Draw a screened subnet design and write the rules for each firewall.

What I should remember

Key Points PDF p.253
  • Screened Subnet Benefits
    • Limits damage from compromised public-facing servers.
    • Enhances network segmentation and security control.