Security Topologies

Networks are different than single servers; networks exist as connections of multiple devices. A key characteristic of a network is its layout, or topology. A proper network topology takes security into consideration and assists in "building security" into the network. Security-related topologies include separating portions of the network by use and function, strategically designing in points to monitor for IDS systems, building in redundancy, and adding fault-tolerant aspects.

Security Zones

The first aspect of security is a layered defense. Just as a castle has a moat, an outside wall, an inside wall, and even a keep, so, too, does a modern secure network have different layers of protection. Different zones are designed to provide layers of defense, with the outermost layers providing basic protection and the innermost layers providing the highest level of protection. A constant issue is that accessibility tends to be inversely related to level of protection, so it is more difficult to provide complete protection and unfettered access at the same time. Trade-offs between access and security are handled through zones, with successive zones guarded by firewalls enforcing ever-increasingly strict security policies. The outermost zone is the Internet, a free area, beyond any specific controls. Between the inner secure corporate network and the Internet is an area where machines are considered at risk. This zone has come to be called the DMZ, after its military counterpart, the demilitarized zone, where neither side has any specific controls. Once inside the inner secure network, separate branches are frequently carved out to provide specific functionality; under this heading, we will discuss intranets, extranets, and virtual LANs (VLANs).


The DMZ is a military term for ground separating two opposing forces, by agreement and for the purpose of acting as a buffer between the two sides. A DMZ in a computer network is used in the same way; it acts as a buffer zone between the Internet, where no controls exist, and the inner secure network, where an organization has security policies in place (see Figure 8-4). To demarcate the zones and enforce separation, a firewall is used on each side of the DMZ. The area between these firewalls is accessible from either the inner secure network or the Internet. Figure 8-4 illustrates these zones as caused by firewall placement. The firewalls are specifically designed to prevent access across the DMZ directly, from the Internet to the inner secure network. Special attention should be paid to the security settings of network devices placed in the DMZ, and they should be considered at all times to be compromised by unauthorized use. A common industry term, hardened operating system, applies to machines whose functionality is locked down to preserve security. This approach needs to be applied to the machines in the

DMZ, and although it means that their functionality is limited, such precautions ensure that the machines will work properly in a less-secure environment.

The idea behind the use of the DMZ topology is to force an outside user to make at least one hop in the DMZ before he can access information inside the trusted network. If the outside user makes a request for a resource from the trusted network, such as a data element from a database via a web page, then this request needs to follow this scenario:

1. A user from the untrusted network (the Internet) requests data via a web page from a web server in the DMZ.

2. The web server in the DMZ requests the data from the application server, which can be in the DMZ or in the inner trusted network.

3. The application server requests the data from the database server in the trusted network.

4. The database server returns the data to the requesting application server.

5. The application server returns the data to the requesting web server.

6. The web server returns the data to the requesting user from the untrusted network.

This separation accomplishes two specific, independent tasks. First, the user is separated from the request for data on a secure network. By having intermediaries do the requesting, this layered approach allows significant security levels to be enforced. Users do not have direct access or control over their requests, and this filtering process can put controls in place. Second, scalability is more easily realized. The multiple-server solution can be made to be very scalable literally to millions of users, without slowing down any particular layer.


The Internet is a worldwide connection of networks and is used to transport e-mail, files, financial records, remote access-you name it-from one network to another. The Internet is not as a single network, but a series of interconnected networks that allow protocols to operate to enable data to flow across it. This means that even if your network doesn't have direct contact with a resource, as long as a neighbor, or a neighbor's neighbor, and so on, can get there, so can you. This large web allows users almost infinite ability to communicate between systems.

Because everything and everyone can access this interconnected web and it is outside of your control and ability to enforce security policies, the Internet should be considered an untrusted network. A firewall should exist at any connection between your trusted network and the Internet. This is not to imply that the Internet is a bad thing-it is a great resource for all networks and adds significant functionality to our computing environments.

The term World Wide Web (WWW) is frequently used synonymously to represent the Internet, but the WWW is actually just one set of services available via the Internet. WWW is more specifically the Hypertext Transfer Protocol (HTTP)-based services that are made available over the Internet. This can include a variety of actual services and content, including text files, pictures, streaming audio and video, and even viruses and worms.


Intranet is a term used to describe a network that has the same functionality as the Internet for users but lies completely inside the trusted area of a network and is under the security control of the system and network administrators. Typically referred to as campus or corporate networks, intranets are used every day in companies around the world. An intranet allows a developer and a user the full set of protocols-HTTP, FTP, instant messaging, and so on-that is offered on the Internet, but with the added advantage of trust from the network security. Content on intranet web servers is not available over the Internet to untrusted users. This layer of security offers a significant amount of control and regulation, allowing users to fulfill business functionality while ensuring security.

Should users inside the intranet require access to information from the Internet; a proxy server can be used to mask the requestor's location. This helps secure the intranet from outside mapping of its actual topology. All Internet requests go to the proxy server. If a request passes filtering requirements, the proxy server, assuming it is also a cache server, looks in its local cache of previously downloaded web pages. If it finds the page in its cache, it returns the page to the requestor without needing to send the request to the Internet. If the page is not in the cache, the proxy server, acting as a client on behalf of the user, uses one of its own IP addresses to request the page from the Internet. When the page is returned, the proxy server relates it to the original request and forwards it on to the user. This masks the user's IP address from the Internet. Proxy servers can perform several functions for a firm; for example, they can monitor traffic requests, eliminating improper requests, such as inappropriate content for work. They can also act as a cache server, cutting down on outside network requests for the same object. Finally, proxy servers protect the identity of internal IP addresses, although this function can also be accomplished through a router or firewall using Network Address Translation (NAT).


An extranet is an extension of a selected portion of a company's intranet to external partners. This allows a business to share information with customers, suppliers, partners, and other trusted groups while using a common set of Internet protocols to facilitate operations. Extranets can use public networks to extend their reach beyond a company's own internal network, and some form of security, typically VPN, is used to secure this channel. The use of the term extranet implies both privacy and security. Privacy is required for many communications, and security is needed to prevent unauthorized use and events from occurring. Both of these functions can be achieved through the use of technologies. Proper firewall management, remote access, encryption, authentication, and secure tunnels across public networks are all methods used to ensure privacy and security for extranets.


Data and voice communications have coexisted in enterprises for decades. Recent connections inside the enterprise of Voice over IP and traditional PBX solutions increase both functionality and security risks. Specific firewalls to protect against unauthorized traffic over telephony connections are available to counter the increased risk.


A local area network (LAN) is a set of devices with similar functionality and similar communication needs, typically co-located and operated off a single switch. This is the lowest level of a network hierarchy and defines the domain for certain protocols at the data link layer for communication. Virtual LANs use a single switch and divide it into multiple broadcast domains and/or multiple network segments, known as trunking. This very powerful technique allows significant network flexibility, scalability, and performance.


Trunking is the process of spanning a single VLAN across multiple switches. A trunk-based connection between switches allows packets from a single VLAN to travel between switches. VLAN 10 is implemented with one trunk and VLAN 20 is implemented by the other. Hosts on different VLANs cannot communicate using trunks and are switched across the switch network. Trunks enable network administrators to set up VLANs across multiple switches with minimal effort. With a combination of trunks and VLANs, network administrators can subnet a network by user functionality without regard to host location on the network or the need to recable machines.

Security Implications

VLANs are used to divide a single network into multiple subnets based on functionality. This permit engineering and accounting, for example, to share a switch because of proximity and yet have separate traffic domains. The physical placement of equipment and cables is logically and programmatically separated so adjacent ports on a switch can reference separate subnets. This prevents unauthorized use of physically close devices through separate subnets, but the same equipment. VLANs also allow a network administrator to define a VLAN that has no users and map all of the unused ports to this

VLAN. Then if an unauthorized user should gain access to the equipment, he will be unable to use unused ports, as those ports will be securely defined to nothing. Both a purpose and a security strength of VLANs is that systems on separate VLANs cannot directly communicate with each other.


Network Address Translation (NAT) uses two sets of IP addresses for resources-one for internal use and another for external (Internet) use. NAT was developed as a solution to the rapid depletion of IP addresses in the IPv4 address space; it has since become an Internet standard (see RFC 1631 for details). NAT is used to translate between the two addressing schemes and is typically performed at a firewall or router. This permits enterprises to use the non-routable private IP address space internally and reduces the number of external IP addresses used across the Internet. Three sets of IP addresses are defined as non-routable, which means that addresses will not be routed across the Internet. These addresses are routable internally and routers can be set to route them, but the routers across the Internet are set to discard packets sent to these addresses. This approach enables a separation of internal and external traffic and allows these addresses to be reused by anyone and everyone who wishes to do so. The three address spaces are:

  • Class A -

  • Class B -

  • Class C -

The use of these addresses inside a network is unrestricted, and they function like any other IP addresses. When outside-that is, Internet-provided-resources are needed for one of these addresses, NAT is required to produce a valid external IP address for the resource. NAT operates by translating the address when traffic passes the NAT device, such as a firewall. The external addresses used are not externally mappable 1:1 to the internal addresses, for this would defeat the purpose of reuse and address-space conservation. Typically, a pool of external IP addresses is used by the NAT device, with the device keeping track of which internal address is using which external address at any given time. This provides a significant layer of security, as it makes it difficult to map the internal network structure behind a firewall and directly address it from the outside. NAT is one of the methods used for enforcing perimeter security by forcing users to access resources through defined pathways such as firewalls and gateway servers.


Tunneling is a method of packaging packets so that they can traverse a network in a secure, confidential manner. Tunneling involves encapsulating packets within packets, enabling dissimilar protocols to coexist in a single communication stream, as in IP traffic routed over an Asynchronous Transfer Mode (ATM) network. Tunneling also can provide significant measures of security and confidentiality through encryption and encapsulation methods. The best example of this is a VPN that is established over a public network through the use of a tunnel; connecting a firm's Boston office to its New York City (NYC) office.

Because of ease of use, low-cost hardware, and strong security, tunnels and the Internet are a combination that will see more use in the future. IPsec, VPN, and tunnels will become a major set of tools for users requiring secure network connections across public segments of networks.