NETWORKS AND INTERNET

 

Introduction

To manage network security, you will need knowledge of how computer networks operate. Those readers who already have a strong working knowledge of network operation may choose to skim or perhaps give it a quick read as review. For others new to computer networking, study will give you a basic introduction to how networks and the internet work. This understanding of networks and internet will be crucial to your comprehension of later topics presented.

We will try to understand that how will examine the basic model of network and the underlying technologies that allow networks to communicate. This information will be the foundation on which all of the other materials in this course are build. You will be able to practice using some utilities i.e. IPCongfig, tracery, & ping.

The OSL Model

Let’s begin with the OSI model or open system interconnect model. This model is a description of  how networks communicate. It describes the various protocols, activities and it delineates how the protocols and activities related to each other. This model is divided into seven layers which is shown in following. It was original developed by the International Standards Organization (ISO).

Layer	Description	Protocols
Application	This layer interfaces directly to the application and performs common application services for the application processes.	Non
Presentation	The presentation layer relieve the application layer of concern regarding syntactical differences in data representation within the end-user system	POP, SMTP, DNS, FTP, Telnet, ARP
Session	The session layer provides the3 mechanism for managing the dialogue between end-user application processes	NetBIOS
Transport	This layer provides end-to-end communication control	TCP
Network	This layer routes the information in the network	IP, ICMP
Data Link	This layer describes the logical organization of data bits transmitted on a particular medium. Data Link is divided into two sub-layers: the Media Access Control layer (MAC) and Logical Link Control layer (LLC)	SLIP, PPP
Physical	This layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals. In other words, the physical layer is the actual NIC, Ethernet Cable and so forth.	None

 

Many networking students memorize this mode. It is good to at least memorize the name of the seven layers and in general understand what they each do. Form a security perspective, the more you understand about network communications, the more sophisticated your defense can be. The most important thing for you to understand is that it describes a hierarchical model of communication. A layer will interact directly with the layer above or below it.

Network Basics

Getting two or more computers and a process to transfer data that is simple in concept but complex in. Consider all the factors involved. First, you'll need to physically connect to the computer. This connection is accomplished by either your computer or plug in a cable to infrared light. The cable is then plugged either directly into another computer or is plugged into a router which ort a hub that will, in turn, connect to several other computers.

Most modern computers have a card called a network interface card or a NIC. The connection is via a cable, so the computer has a connection slot that looks like a telephone jack, just a little bigger that looks part of the NIC that is external. Of course wireless network, which is a slot for a cable to connect to, the wireless network simply uses infrared signals to transmit to a nearby wireless router or hub.

Media Access Control (MAC) Addresses

MAC addresses are an interesting topic. (The data link layer of the OSI model of Mac you can feel Layer). A MAC  address is a unique address for a NIC. Every NIC in the world has a unique address represented by a six byte hexadecimal number. MAC addresses is a protocol that is used to change the IP addresses. This protocol is the Address Resolution Protocol or ARP. Therefore, when you type in a Web address, the DNS (Domain Name Server) protocol is used to translate that into a IP address. The ARP protocol will then translate that IP address into a specific MAC address of an individual NIC.

DNS Servers

How does a URL get translated into a IP address? IP is the URL that the computer how to do? Servers have been set up to perform this task. There are only set up servers to do the job. DNS stands for Domain Name Server (or System or Service). DNS translates domain names (www.example.com) into IP address (199.246.58.4). To remember domain names are alphabetic, because they are easy, but the Internet is really based on IP addresses. Thus, every time you use a domain name, a DNS server must translate the name into the corresponding IP address. If you are on corporate network, you probably have a DNS server on your network. If not, those, you ISP have one. These servers maintain a table of IP-to URL entries.

From time to time there are transfers of DNS data, called zone transfers, that allow one DNS server to send its changes to another. Across the Internet, there are root DNS server that are maintained with centralized data for all registered URL/IP addresses. The DNS system is, in fact, its own network. If one DNS server does not know how to translate a particular domain name, it asks another one and so on until the correct IP address is returned.

Primary DNS is the name given to the server or service that holds the authoritative information for a domain. Actually, a DNS server ( the computer / software) is not specifically “primary” or “Secondary”. A DNS server can be primary for one zone (domain) and secondary for another. By definition, a primary DNS server hold the master copy of the data for a zone and secondary servers have copies of this data that they synchronize with the primary server through zone transfers at intervals or when prompted by the primary server.

The physical connection: Local Network

Cable is a way through which more than one computes connected. The cable connection used with hard-wired NICs us an RJ-45 connection. (RJ is short for “Registered Jack”, which is an international industry standard). In contrast to the computer’s RJ 45 jacks, standard telephonic lines use RJ 11 jacks. The biggest different between jack involves the number of wires in the connector, also called the terminator. Phone lines have four wires, whereas RJ 45 connectors has eight.

If you look on the back side of computers or laptop, you will mostly find three ports that, at first glance, look as phone jacks two of the three ports are probably for a traditional modern and telephone and accept a standard RJ-11 jack.  One port is used for RJ 45 connector. Not all computers come with a NIC, most modern computer do. Additionally, many modern computers no longer contain an internal modern, in which case there would not be an RJ 11 jack.

This standard connector jack must be crimped on the end of the cable. The cable used in most networks today is a category 5 cables – or CAT 5, as it is commonly known. (Not that CAT-6 cable is becoming more prevalent with high – speed networks). 

 

Category	Specifications	Uses
1	Low-speed analog (less than 1 MHz)	Telephone, doorbell
2	Analog line (less than 10 MHz)	Telephone
3	Up to 16 MHz or 10 MBps (megabits per second)	Voice transmissions
4	Up to 20 MHz / 16 MBps	Data lines, Ethernet networks
5	100 MHz / 100 MBps	Most common types network cable
6	250 MHz / 1000 MBps	Very high-speed networks

 

The type of cable used in connecting computers is also often referred to as shielded twisted pair cable (UTP). In UTP, the wires in the cable are in pairs, twist together without any additional shielding. As you can see in above mentioned table, each subsequent category of cable is somewhat faster and more robust than the last. It should be noted that, although CAT-4 can be used for networks, it is most never used for that purpose simply because it is slower, less reliable and oldest technology. You will usually see CAT – 5 cables and it is also increase upto CAT – 6.

Notice the speeds listed in table, such as MBps. This speed stands for megabits per second. Ultimately, everything in computer is stored in binary format using a 1 or 0. These units are called bits. It takes eight bits, or one byte, to represent a single character such as a letter, number or carriage return. It follows, then, that CAT – 5 cable can transmit up to 100,000,000 bits per second. This is known as the bandwidth of the cable. Remember, though, that this is the maximum that can be transmitted “across the wire” at any given second. If multiple users are on a network and all of them are sending data, the traffic generated is going to quickly use up all of the bandwidth. Simple scanned-in photos can easily reach two megabytes or much more. Steaming media, such as video is perhaps the most demanding on bandwidth.

If you simply want to connect two computers to each other, you can have the cable go directly from one computer to the other. But what do you do if you wish to connect more than one computer? What if 100 computers need to be connected on a network? There are three devices that can help you accomplish this task: the hub, the switch, and the router. These devices each use CAT – 5 or CAT – 6 cable with RJ – 45 connectors. 

1.      The Hub
the simplest connection device is the HUB. A hub is a small, box-shaped electronic device into which you can plug network cable. It will have four or more (typically 24) RJ 45 jack him, met each port. This is as a center as many ports can connect to the computer. You can also connect on hub to another; this strategy is referred to as “stacking” hub. Very cheap and easy plug-in cable to connect --- centers. However, however, hubs have a downside. If you send a packet from one computer to another, a copy of that packet is actually send out from every port on the hub. All of these copies lead to a great deal of unnecessary network traffic. There is no way of knowing where the center, because it is a very simple device occurs, a packet should be. Therefore, it simply sends copies of the packet out of all of its ports.

2.      The Switch
The next connection device option is known as Switch. A switch is basically an intelligent hub. However, a switch receive a packet, it will send that packet only out the port for the computer to which it needs to go. A switch builds a table based on MAC addresses and uses that to determine where a packet is being sent. How this determination is made is explained in the Data Transmission section below.

3.      The Router
Finally, if you want to connect two or more networks together, you use a router. A router is similar in concept to a hub or switch, as it does relay packets; yet, it is far more sophisticated. You can program most routers and control how they relay pockets. How your router vendor programs such details are different form. However, you should be aware that most routers are programmable, allowing you to change how they route traffic. Moreover, unlike using a hub or switch, a router connected to two networks are still separate networks. In short, the three basic connection devices are the hub, switch and router all of which connect category 5 or category 6 cable using RJ 45 connectors.