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Network Design Elements and Components

As you create a network security policy, you must define procedures to defend your network and users against harm and loss. With this objective in mind, a network design and the included components play an important role in implementing the overall security of the organization.

An overall security solution includes design elements and components such as firewalls, VLANS, and perimeter network boundaries that distinguish between private networks, intranets, and the Internet. This section discusses these elements and will help you tell them apart and understand their function in the security of the network.

Demilitarized Zone

A demilitarized zone (DMZ) is a small network between the internal network and the Internet that provides a layer of security and privacy. Both internal and external users may have limited access to the servers in the DMZ. Figure 3.3 depicts a DMZ.

Figure 3.3

Figure 3.3 A DMZ.

Often, web and mail servers are placed in the DMZ. Because these devices are exposed to the Internet, it is important that they are hardened and patches are kept current. Table 3.2 lists the most common services and ports that are run on servers inside the DMZ.

Table 3.2. Commonly Used Ports on Servers in the DMZ

















The DMZ is an area that allows external users to access information that the organization deems necessary but will not compromise any internal organizational information. This configuration allows outside access, yet prevents external users from directly accessing a server that holds internal organizational data.


An intranet is a portion of the internal network that uses web-based technologies. The information is stored on web servers and accessed using browsers. Although web servers are used, they don’t necessarily have to be accessible to the outside world. This is possible because the IP addresses of the servers are reserved for private, internal use. You learn more about private IP addresses in the “NAT” section, later in this chapter. If the intranet can be accessed from public networks, it should be through a virtual private network (VPN) for security reasons. VPNs are described in greater detail in Chapter 6, “Securing Communications.”


An extranet is the public portion of the company’s IT infrastructure that allows resources to be used by authorized partners and resellers that have proper authorization and authentication. This type of arrangement is commonly used for business-to-business relationships. Because an extranet can provide liability for a company, care must be taken to ensure that VPNs and firewalls are configured properly and that security policies are strictly enforced.

Virtual Local Area Network

The purpose of a virtual local area network (VLAN) is to unite network nodes logically into the same broadcast domain regardless of their physical attachment to the network. VLANs provide a way to limit broadcast traffic in a switched network. This creates a boundary and, in essence, creates multiple, isolated LANs on one switch. Because switches operate on Layer 2 (data link layer) of the OSI model, a router is required if data is to be passed from one VLAN to another.

Frame tagging is the technology used for VLANs. The 802.1Q standard defines a mechanism that encapsulates the frames with headers, which then tags them with a VLAN ID. VLAN-aware network devices look for these tags in frames and make appropriate forwarding decisions. A VLAN is basically a software solution that allows creating unique tag identifiers to be assigned to different ports on the switch.

The most notable benefit of using a VLAN is that it can span multiple switches. Because users on the same VLAN don’t have to be associated by physical location, they can be grouped by department or function. Here are the benefits that VLANs provide:

  • Users can be grouped by department rather than physical location.
  • Moving and adding users is simplified. No matter where a user physically moves, changes are made to the software configuration in the switch.
  • Because VLANs allow users to be grouped, applying security policies becomes easier.

Keep in mind that use of a VLAN is not an absolute safeguard against security infringements. It does not provide the same level of security as a router. A VLAN is a software solution and cannot take the place of a well subnetted or routed network. It is possible to make frames hop from one VLAN to another. This takes skill and knowledge on the part of an attacker, but it is possible. For more information about frame tagging and VLANs, see the “Suggested Reading and Resources” section at the end of the chapter.

Network Address Translation

Network Address Translation (NAT) acts as a liaison between an internal network and the Internet. It allows multiple computers to connect to the Internet using one IP address. An important security aspect of NAT is that it hides the internal network from the outside world. In this situation, the internal network uses a private IP address. Special ranges in each IP address class are used specifically for private addressing. These addresses are considered nonroutable on the Internet.

Here are the private address ranges:

  • Class A— network. Valid host IDs are from to
  • Class B— through networks. Valid host IDs are from through
  • Class C— network. Valid host IDs are from to

For smaller companies, NAT can be used in the form of Windows Internet Connection Sharing (ICS), where all machines share one Internet connection, such as a dial-up modem. NAT can also be used for address translation between multiple protocols, which improves security and provides for more interoperability in heterogeneous networks.


Subnetting can be done for several reasons. If you have a Class C address and 1,000 clients. you will have to subnet the network or use a custom subnet mask to accommodate all the hosts. The most common reason networks are subnetted is to control network traffic. Splitting one network into two or more and using routers to connect each subnet together means that broadcasts can be limited to each subnet. However, often networks are subnetted to improve network security, not just performance. Subnetting allows you to arrange hosts into the different logical groups that isolate each subnet into its own mini network. Subnet divisions can be based on business goals and security policy objectives. For example, perhaps you use contract workers and want to keep them separated from the organizational employees. Often, organizations with branches use subnets to keep each branch separate. When your computers are on separate physical networks, you can divide your network into subnets that enable you to use one block of addresses on multiple physical networks. If an incident happens and you notice it quickly, you can usually contain the issue to that particular subnet.

Notice that the 127 network address is missing. Although the network is in technically in the Class A area, using addresses in this range causes the protocol software to return data without sending traffic across a network. For example, the address is used for TCP/IP loopback testing, and the address is used by most DNS black lists for testing purposes. Should you need additional review on IP addressing and subnetting, a wide variety of information is available. One such website is Learntosubnet.com. Figure 3.4 shows an internal network with two different subnets. Notice the IP addresses, subnet masks, and default gateway.

Figure 3.4

Figure 3.4 A segmented network. Notice the subnets and identified next to the router. These are not valid IP addresses for a network router and are used to identify the 192.168.1.x and 192.168.2.x networks in routing tables.

IPv6 is designed to replace IPv4. Addresses are 128 bits rather than the 32 bits used in IPv4. Just as in IPv4, blocks of addresses are set aside in IPv6 for private addresses. In IPv6, internal addresses are called unique local addresses (ULA). Addresses starting with fe80: are called link-local addresses and are routable only in the local link area. IPv6 addresses are represented in hexadecimal. For more information about IPv6, visit http://www.ipv6.org/.

Network Interconnections

Besides securing ports and protocols from outside attacks, connections between interconnecting networks should be secured. This situation may come into play when an organization establishes network interconnections with partners. This might be in the form of an extranet or actual connection between the involved organizations as in a merger, acquisition, or joint project. Business partners can include government agencies and commercial organizations. Although this type of interconnection increases functionality and reduces costs, it can result in security risks. These risks include compromise of all connected systems and any network connected to those systems, along with exposure of data the systems handle. With interconnected networks, the potential for damage greatly increases because one compromised system on one network can easily spread to other networks.

Organizational policies should require an interconnection agreement for any system or network that shares information with another external system or network. Organizations need to carefully evaluate risk-management procedures and ensure that the interconnection is properly designed. The partnering organizations have little to no control over the management of the other party’s system, so without careful planning and assessment, both parties can be harmed. National Institute of Standards and Technology (NIST) Special Publication (SP) 800-47, Security Guide for Interconnecting Information Technology Systems, provides guidance for any organization that is considering interconnecting with a government agency or other organization.

Network Access Control

One the most effective ways to protect the network from malicious hosts is to use network access control (NAC). NAC offers a method of enforcement that helps ensure computers are properly configured. The premise behind NAC is to secure the environment by examining the user’s machine and based on the results grant (or not grant) access accordingly. It is based on assessment and enforcement. For example, if the user’s computer patches are not up-to-date, and no desktop firewall software is installed, you can decide whether to limit access to network resources. Any host machine that doesn’t comply with your defined policy could be relegated to remediation server, or put on a guest VLAN. The basic components of NAC products are

  • Access requestor (AR)—This is the device that requests access. The assessment of the device can be self-performed or delegated to another system.
  • Policy decision point (PDP)—This is the system that assigns a policy based on the assessment. The PDP determines what access should be granted and may be the NAC’s product-management system.
  • Policy enforcement point (PEP)—This is the device that enforces the policy. This device may be a switch, firewall, or router.

The four ways NAC systems can be integrated into the network are

  • Inline—An appliance in the line, usually between the access and the distribution switches
  • Out-of-band—Intervenes and performs an assessment as hosts come online and then grants appropriate access
  • Switch based—Similar to inline NAC except enforcement occurs on the switch itself
  • Host based—Relies on an installed host agent to assess and enforce access policy

In addition to providing the ability to enforce security policy, contain noncompliant users, and mitigate threats, NAC offers a number of business benefits. The business benefits include compliance, a better security posture, and operational cost management.


The transmission of data through equipment in a telecommunications environment is known as telephony. Telephony includes transmission of voice, fax, or other data. This section describes the components that need to be considered when securing the environment. Often, these components are neglected because they are not really network components. However, they use communications equipment that is susceptible to attack and therefore must be secured.


The telecommunications (telecom) system and Private Branch Exchange (PBX) are a vital part of an organization’s infrastructure. Besides the standard block, there are also PBX servers, where the PBX board plugs into the server and is configured through software on the computer. Many companies have moved to Voice over IP (VoIP) to integrate computer telephony, videoconferencing, and document sharing.

For years PBX-type systems have been targeted by hackers, mainly to get free long-distance service. The vulnerabilities that phone networks are subject to include social engineering, long-distance toll fraud, and breach of data privacy.

To protect your network, make sure the PBX is in a secure area, any default passwords have been changed, and only authorized maintenance is done. Many times, hackers can gain access to the phone system via social engineering because this device is usually serviced through a remote maintenance port.

Voice over Internet Protocol

VoIP uses the Internet to transmit voice data. A VoIP system might be composed of many different components, including VoIP phones, desktop systems, PBX servers, and gateways. VoIP PBX servers are susceptible to the same type of exploits as other network servers. These attacks include DoS and buffer overflows, with DoS being the most prevalent. In addition, there are voice-specific attacks and threats. H.323 and Inter Asterisk eXchange (IAX) are specifications and protcols for audio/video. They enable VoIP connections between servers and enable client/server communication. H.323 and IAX protocols can be vulnerable to sniffing during authentication. This allows an attacker to obtain passwords that may be used to compromise the voice network. Session Initiation Protocol (SIP) is commonly used in instant messaging, but it can also be used as an alternative for VoIP. Using SIP can leave VoIP networks open to unauthorized transport of data. Man-in-the-middle attacks between the SIP phone and SIP proxy allow the audio to be manipulated, causing dropped, rerouted, or playback calls. Many components comprise a VoIP network, and VoIP security is built upon many layers of traditional data security. Therefore, access can be gained in a lot of areas.

Implementing the following solutions can help mitigate the risks and vulnerabilities associated with VoIP:

  • Encryption
  • Authentication
  • Data validation
  • Nonrepudiation


Modems are used via the phone line to dial in to a server or computer. They are gradually being replaced by high-speed cable and Digital Subscriber Line (DSL) solutions, which are faster than dial-up access. However, some companies still use modems for employees to dial into the network and work from home. The modems on network computers or servers are usually configured to take incoming calls. Leaving modems open for incoming calls with little to no authentication for users dialing in can be a clear security vulnerability in the network. For example, war-dialing attacks take advantage of this situation. War-dialing is the process by which an automated software application is used to dial numbers in a given range to determine whether any of the numbers are serviced by modems that accept dial-in requests. This attack can be set to target connected modems that are set to receive calls without any authentication, thus allowing attackers an easy path into the network. You can resolve this problem area in several ways:

  • Set the callback features to have the modem call the user back at a preset number.
  • Make sure authentication is required using strong passwords.
  • Be sure employees have not set up modems at their workstations with remote-control software installed.

Cable and DSL modems are popular these days. They act more like routers than modems. Although these devices are not prone to war-dialing attacks, they do present a certain amount of danger by maintaining an always-on connection. If you leave the connection on all the time, a hacker has ample time to get into the machine and the network. The use of encryption and firewall solutions will help keep the environment safe from attacks.

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