computer information

Ethernet Switching

2009-02-10T15:29:00.000-08:00

Shared Ethernet works extremely well under ideal conditions. If the number of devices that try to access the network is low, the number of collisions stays well within acceptable limits. However, when the number of users on the network increases, the number of collisions can significantly reduce performance. Bridges were developed to help correct performance problems that arose from increased collisions. Switches evolved from bridges to become the main technology in modern Ethernet LANs.

Collisions and broadcasts are expected events in modern networks. They are engineered into the design of Ethernet and higher layer technologies. However, when collisions and broadcasts occur in numbers that are above the optimum, network performance suffers. Collision domains and broadcast domains should be designed to limit the negative effects of collisions and broadcasts.

Cabling LANs

2009-02-06T18:17:00.000-08:00

Even though each LAN is unique, there are many design aspects that are common to all LANs. For example, most LANs follow the same standards and use the same components. This module presents information on elements of Ethernet LANs and common LAN devices. There are several types of WAN connections. They range from dial-up to broadband access and differ in bandwidth, cost, and required equipment.

Various symbols are used to represent media types. Token Ring is represented by a circle. FDDI is represented by two concentric circles and the Ethernet symbol is represented by a straight line. Serial connections are represented by a lightning bolt.

Each computer network can be built with many different media types. The function of media is to carry a flow of information through a LAN. Wireless LANs use the atmosphere, or space, as the medium. Other networking media confine network signals to a wire, cable, or fiber. Networking media are considered Layer 1, or physical layer, components of LANs.

Each type of media has advantages and disadvantages. These are based on the following factors:

• Cable length
• Cost
• Ease of installation
• Susceptibility to interference

Cisco Catalyst® 2950-24

2009-01-30T03:06:00.000-08:00

The Cisco Catalyst® 2950-24 is a member of the Cisco Catalyst 2950 Series switches, and is a standalone, fixed-configuration, managed 10/100 switch providing user connectivity for small to mid-sized networks. This wire-speed desktop switch comes with Standard Image (SI) software features and offers Cisco IOS® functionality for basic data, video and voice services at the edge of the network. Embedded in all Catalyst 2950 Series is the Cisco Cluster Management Suite (CMS) Software, which allows users to simultaneously configure and troubleshoot multiple Catalyst desktop switches using a standard Web browser.

Coming Soon : Microsoft Office 14

2009-01-25T05:27:00.000-08:00

Microsoft Office 14 ("Office 14" for short) is the working title for the next version of the Microsoft Office System productivity suite for Microsoft Windows. It entered development during 2006 while Microsoft was finishing work on Microsoft Office 12, which was released as the 2007 Microsoft Office System. The major version number 13 has been skipped, presumably due to aversion to the number 13.

It was previously thought that Office 14 would ship in the first half of 2009, but more recent information suggests a late 2009/early 2010 release timeframe. Microsoft Office 14 is expected to be released around the time of Windows 7. According to an article published in InfoWorld in April 2006, Office 14 will be more "role-based" than previous versions.The article cites Simon Witts, corporate vice president for Microsoft's Enterprise and Partner Group, as claiming that there would be features tailored to employees in "roles such as research and development professionals, sales persons, and human resources." Borrowing from ideas termed "Web 2.0" when implemented on the Internet, it is likely that Microsoft will incorporate features of SharePoint Server in Office 14.
Office 14 will implement the ISO compliant version of Office Open XML which was standardized as ISO 29500 in March 2008. Microsoft plans to offer a Web-based version of its Office productivity suite, known as Office Web, that will debut with the release of Office 14. Office Web will include online versions of Word, Excel, PowerPoint and OneNote.
The next versions of Microsoft Office Visio, OneNote and Microsoft Office Project will feature the Ribbon interface element used in other Office 2007 applications.
On January 15, 2009, screenshots of an Office 14 alpha build were leaked by a tester. A beta of Office 14 is expected in May, with the final version being slated for the end of 2009.

Filters and access lists

2009-01-25T03:43:00.000-08:00

Regulating which packets can go between two sites is a fairly simple concept on the surface- it shouldn't be and isn't difficult for any router or firewall to decide simply not to forward all packets from a particular site. Unfortunately, the reason most people connect to the Internet is so that they may exchange packets with remote sites. Developing a plan that allows the right packets through at the right time and denies the malicious packets is a thorny task which is far beyond this article's scope. A few basic techniques are worth discussing, however.

• Restricting access in, but not out: Almost all packets (besides those at the lowest levels which deal with network reachability) are sent to destination sockets of either UDP or TCP. Typically, packets from remote hosts will attempt to reach one of what are known as the well known ports. These ports are monitored by applications which provide services such as Mail Transfer and Delivery, Usenet News, the time, Domain Name Service, and various login protocols. It is trivial for modern routers or firewalls only to allow these types of packets through to the specific machine that provides a given service. Attempts to send any other type of packet will not be forwarded. This protects the internal hosts, but still allows all packets to get out. Unfortunately this isn't the panacea that it might seem.

• The problem of returning packets: Let's pretend that you don't want to let remote users log into your systems unless they use a secure, encrypting application such as S/Key. However, you are willing to allow your users to attempt to connect to remote sites with telnet or ftp. At first glance, this looks simple: you merely restrict remote connections to one type of packet and allow any type of outgoing connection. Unfortunately, due to the nature of interactive protocols, they must negotiate a unique port number to use once a connection is established. If they didn't, at any given time, there could only be one of each type of interactive session between any given two machines. This results in a dilemma: all of a sudden, a remote site is going to try to send packets destined for a seemingly random port. Normally, these packets would be dropped. However, modern routers and firewalls now support the ability to dynamically open a small window for these packets to pass through if packets have been recently transmitted from an internal host to the external host on the same port. This allows connections that are initiated internally to connect, yet still denies external connection attempts unless they are desired.

• Dynamic route filters: A relatively recent technique is the ability to dynamically add entire sets of route filters for a remote site when a particular set of circumstances occur. With these techniques, it is possible to have a router automatically detect suspicious activity (such as ISS or SATAN) and deny a machine or entire site access for a short time. In many cases this will thwart any sort of automated attack on a site.

Filters and access lists are typically placed on all three types of systems, although they are most common on routers.

Packet Level Encryption

2009-01-24T04:36:00.000-08:00

Another approach is to encrypt traffic at a higher layer in the TCP/IP stack. Several methods exist for the secure authentication and encryption of telnet and rlogin sessions (Kerberos, S/Key and DESlogin) which are examples of encryption at the highest level of the stack (the application layer). The advantages to encrypting traffic at the higher layer are that the processor overhead of dealing with a VPN is eliminated, inter-operability with current applications is not affected, and it is much easier to compile a client program that supports application layer encryption than to build a VPN. It is possible to encrypt traffic at essentially any of the layers in the IP stack.

Particularly promising is encryption that is done at the TCP level which provides fairly transparent encryption to most network applications. It is important to note that both of these methods can have performance impacts on the hosts that implement the protocols, and on the networks which connect those hosts. The relatively simple act of encapsulating or converting a packet into a new form requires CPU-time and uses additional network capacity. Encryption can be a very CPU-intensive process and encrypted packets may need to be padded to uniform length to guarantee the robustness of some algorithms. Further, both methods have impacts on other areas (security related and otherwise- such as address allocation, fault tolerance and load balancing) that need to be considered before any choice is made as to which is best for a particular case.

Network Security, Filters and Firewalls Part two

2009-01-23T18:08:00.000-08:00

Within the last five years, businesses have begun to need to share data across wide areas. This has prompted efforts to convert principally LAN-based protocols into WAN-friendly protocols. The result has spawned an entire industry of consultants who know how to manipulate routers, gateways and networks to force principally broadcast protocols across point-to-point links (two very different methods of transmitting packets across networks). Recently (within the last 2 or 3 years) more and more companies have realized that they need to settle on a common networking protocol. Frequently the protocol of choice has been TCP/IP, which is also the primary protocol run on the Internet. The emerging ubiquitousness of TCP/IP allows companies to interconnect with each other via private networks as well as through public networks.

This is a very rosy picture: businesses, governments and individuals communicating with each other across the world. While reality is rapidly approaching this utopian picture, several relatively minor issues have changed status from low priority to extreme importance. Security is probably the most well known of these problems. When businesses send private information across the net, they place a high value on it getting to its destination intact and without being intercepted by someone other than the intended recipient. Individuals sending private communications obviously desire secure communications. Finally, connecting a system to a network can open the system itself up to attacks. If a system is compromised, the risk of data loss is high.

It can be useful to break network security into two general classes:
• methods used to secure data as it transits a network
• methods which regulate what packets may transit the network

While both significantly effect the traffic going to and from a site, their objectives are quite different.

Network Security, Filters and Firewalls Part One

2009-01-23T17:51:00.000-08:00

This article is a general introduction to network security issues and solutions in the Internet; emphasis is placed on route filters and firewalls. It is not intended as a guide to setting up a secure network; its purpose is merely as an overview. Some knowledge of IP networking is assumed, although not crucial. In the last decade, the number of computers in use has exploded. For quite some time now, computers have been a crucial element in how we entertain and educate ourselves, and most importantly, how we do business. It seems obvious in retrospect that a natural result of the explosive growth in computer use would be an even more explosive (although delayed) growth in the desire and need for computers to talk with each other.

The growth of this industry has been driven by two separate forces which until recently have had different goals and end products. The first factor has been research interests and laboratories; these groups have always needed to share files, email and other information across wide areas. The research labs developed several protocols and methods for this data transfer, most notably TCP/IP. Business interests are the second factor in network growth. For quite some time, businesses were primarily interested in sharing data within an office or campus environment, this led to the development of various protocols suited specifically to this task.

Wide Area Network (WAN)

2009-01-21T23:59:00.000-08:00

Wide Area Network (WAN) is a computer network that covers a broad area (i.e., any network whose communications links cross metropolitan, regional, or national boundaries). Contrast with personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area (e.g., a city) respectively.

The largest and most well-known example of a WAN is the Internet.
WANs are used to connect LANs and other types of networks together, so that users and computers in one location can communicate with users and computers in other locations. Many WANs are built for one particular organization and are private. Others, built by Internet service providers, provide connections from an organization's LAN to the Internet. WANs are often built using leased lines. At each end of the leased line, a router connects to the LAN on one side and a hub within the WAN on the other. Leased lines can be very expensive.

Instead of using leased lines, WANs can also be built using less costly circuit switching or packet switching methods. Network protocols including TCP/IP deliver transport and addressing functions. Protocols including Packet over SONET/SDH, MPLS, ATM and Frame relay are often used by service providers to deliver the links that are used in WANs. X.25 was an important early WAN protocol, and is often considered to be the "grandfather" of Frame Relay as many of the underlying protocols and functions of X.25 are still in use today (with upgrades) by Frame Relay.

Routing Components

2009-01-21T23:04:00.000-08:00

Routing involves two basic activities: determining optimal routing paths and transporting information groups (typically called packets) through an internetwork. In the context of the routing process, the latter of these is referred to as packet switching. Although packet switching is relatively straightforward, path determination can be very complex.

Path Determination

Routing protocols use metrics to evaluate what path will be the best for a packet to travel. A metric is a standard of measurement, such as path bandwidth, that is used by routing algorithms to determine the optimal path to a destination. To aid the process of path determination, routing algorithms initialize and maintain routing tables, which contain route information. Route information varies depending on the routing algorithm used.
Routing algorithms fill routing tables with a variety of information. Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router representing the "next hop" on the way to the final destination. When a router receives an incoming packet, it checks the destination address and attempts to associate this address with a next hop. Figure 5-1 depicts a sample destination/next hop routing table.

Routing Basics

2009-01-21T22:55:00.000-08:00

This chapter introduces the underlying concepts widely used in routing protocols. Topics summarized here include routing protocol components and algorithms. In addition, the role of routing protocols is briefly contrasted with the role of routed or network protocols. Subsequent chapters in Part VII, "Routing Protocols," address specific routing protocols in more detail, while the network protocols that use routing protocols are discussed in Part VI, "Network Protocols."

What Is Routing?

Routing is the act of moving information across an internetwork from a source to a destination. Along the way, at least one intermediate node typically is encountered. Routing is often contrasted with bridging, which might seem to accomplish precisely the same thing to the casual observer. The primary difference between the two is that bridging occurs at Layer 2 (the link layer) of the OSI reference model, whereas routing occurs at Layer 3 (the network layer). This distinction provides routing and bridging with different information to use in the process of moving information from source to destination, so the two functions accomplish their tasks in different ways.

The topic of routing has been covered in computer science literature for more than two decades, but routing achieved commercial popularity as late as the mid-1980s. The primary reason for this time lag is that networks in the 1970s were simple, homogeneous environments. Only relatively recently has large-scale internetworking become popular.