What is a computer virus?
A computer virus is a program designed to spread itself by first infecting executable files or the system areas of hard and floppy disks and then making copies of itself. Viruses usually operate without the knowledge or desire of the computer user.

What kind of files can spread viruses?
Viruses have the potential to infect executable code, not just program files. Some viruses infect executable code in the boot sector of floppy disks or in
system areas of hard drives. Another type of virus, known as a 'macro' virus, can infect word processing and spreadsheet documents that use macros. And it's possible for HTML documents containing JavaScript or other executable code to spread viruses or malicious code.

Since virus code must be executed to have any effect, files that the computer treats as pure data are safe. This includes graphics and sound files such as .gif, .jpg, .mp3, .wav, etc., as well as plain text in .txt files. For example, just viewing picture files won't infect your computer with a virus. The virus code has to be in a form of an .exe program file or a Word .doc file, that the computer will actually try to execute.

How do viruses spread?
When you execute infected program code, the virus code will run and attempt to infect other programs, either on the same computer or on other computers connected over a network . The newly infected programs will then try to infect yet more programs, and the pattern repeats itself.

When you share a copy of an infected file with other computer users,running the file may also infect their computers; and files from those computers may spread the infection to yet more computers.

If your computer is infected with a boot sector virus, the virus tries to write copies of itself to the system areas of floppy disks and hard disks. Then the infected floppy disks may infect other computers that boot from them, and the virus copy on the hard disk will try to infect still more
floppies.

Some viruses, known as 'multipartite' viruses, can spread both by infecting files and by infecting the boot areas of floppy disks.

What do viruses do to computers?
Viruses are software programs, and they can do the same things as any other programs running on a computer. The actual effect of any particular virus depends on how it was programmed by the person who wrote the virus.

Some viruses are deliberately designed to damage files or otherwise interfere with your computer's operation, while others don't do anything but try to spread themselves around. But even the ones that just spread themselves are harmful, since they damage files and may cause other problems in the process of spreading. Note that viruses can't do any damage to hardware: Warnings about viruses that will physically destroy your computer are usually hoaxes, not legitimate virus warnings.

What is a Trojan horse program?
A type of program that is often confused with viruses is a 'Trojan horse' program. This is not a virus, but simply a program (often harmful) that pretends to be something else. For example, you might download what you think is a new game; but when you run it, it deletes files on your hard drive. Or the third time you start the game, the program E-mails your saved passwords to another person. Note: simply downloading a file to your computer won't activate a virus or Trojan horse; you have to execute the code in the file to trigger it. This could mean running a program file, or opening a Word/Excel document in a program (such as Word or Excel) that can execute any macros in the document.

What about viruses in E-mail?
You can't get a virus just by reading a plain-text E-mail message or Usenet post. What you have to watch out for are encoded messages containing embedded executable code (i.e., JavaScript in an HTML message) or messages that include an executable file attachment (i.e., an encoded program file or a Word document containing macros). In order to activate a virus or Trojan horse program, your computer has to execute some type of code. This could be a program attached to an E-mail, a Word document you downloaded from the Internet, or something received on a floppy disk. There's no special hazard in files attached to Usenet posts or E-mail messages: they're no more dangerous than any other file.

What can I do to reduce the chance of getting viruses from E-mail?
Treat any file attachments that might contain executable code as carefully as you would any other new files: save the attachment to disk and then check it with an up-to-date virus scanner before opening the file. If your E-mail or news software has the ability to automatically execute JavaScript, Word macros, or other executable code contained in or attached to a message, we strongly recommend that you disable this feature. If an executable file shows up unexpectedly attached to an E-mail, you should delete it unless you can positively verify what it is, who it came from, and why it was sent to you.

The recent outbreak of the Melissa virus was a vivid demonstration of the need to be extremely careful when you receive E-mail with attached files or documents. Just because an E-mail appears to come from someone you trust, this does NOT mean the file is safe or that the supposed sender had anything to do with it.

Some general tips on avoiding virus infections:
1. Install anti-virus software from a well-known, reputable company. Update it regularly, and use it regularly. New viruses come out every single day; an a-v program that hasn't been updated for several months will not provide much protection against current viruses.

2. In addition to scanning for viruses on a regular basis, install an 'on access' scanner (included in most good a-v software packages) and configure it to start automatically each time you boot your system. This will protect your system by checking for viruses each time your computer accesses an executable file.

3. Virus scan any new programs or other files that may contain executable code before you run or open them, no matter where they come from. There have been cases of commercially distributed floppy disks and CD-ROMs spreading virus infections.

4. Anti-virus programs aren't very good at detecting Trojan horse programs, so be extremely careful about opening binary files and Word/Excel documents from unknown or 'dubious' sources. This includes posts in binary newsgroups, downloads from web/ftp sites that aren't well-known or don't have a good reputation, and executable files unexpectedly received as attachments to E-mail or during an on-line chat session.

5. If your E-mail or news software has the ability to automatically execute JavaScript, Word macros, or other executable code contained in or attached to a message, I strongly recommend that you disable this feature.

6. Be extremely careful about accepting programs or other files during on-line chat sessions: this seems to be one of the more common means that people wind up with virus or Trojan horse problems. And if any other family members (especially younger ones) use the computer, make sure they know not to accept any files while using chat.

7. Do regular backups. Some viruses and Trojan horse programs will erase or corrupt files on your hard drive, and a recent backup may be the only way to recover your data.

Ideally, you should back up your entire system on a regular basis. If this isn't practical, at least backup files that you can't afford to lose or that would be difficult to replace: documents, bookmark files, address books, important E-mail, etc.

Dealing with virus infections:
Just because your computer is acting strangely or one of your programs doesn't work right, this does NOT mean that your computer has a virus. Drastic measures such as formatting your hard drive or using FDISK should be avoided. They are frequently useless at cleaning a virus infection, and may do more harm than good unless you're very knowledgeable about the effects of the particular virus you're dealing with.

1. If you haven't used a good, up-to-date anti-virus program on your computer, do that first. Many problems blamed on viruses are actually caused by software configuration errors or other problems that have nothing to do with a virus.

2. If you do get infected by a virus, follow the directions in your anti-virus program for cleaning it. If you have backup copies of the infected files, use those to restore the files. Check the files you restore to make sure your backups weren't infected.

For assistance, check the web site and support services for your anti-virus software.

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IP Addressing
An IP (Internet Protocol) address uniquely identifies a node or host connection to an IP network. System administrators or network designers assign IP addresses to nodes and hosts. An IP address is a 32 bit binary number represented as 4 fields each representing 8 bit numbers in the range 0 to 255, (called octets) separated by decimal points. For example: 150.215.17.9 It is sometimes useful to view the values in their binary form.

150. 215. 17. 9
10010110.11010111.00010001.00001001

An IP address consists of two parts. One identifies the network and one identifies the node. The Class of the address determines which part belongs to the network address and which part belongs to the node address.

Address classes
There are 5 different address classes. The first byte of the first octet determines the class of the address.
Class A addresses start with 0.
Class B addresses start with 10.
Class C addresses start with 110.
Class D addresses start with 1110.
Class E addresses start with 1111.

Classes can also be distinguished in decimal notation. If the first octet is between:
1 and 126 it is a Class A address.
128 and 191 it is a Class B address
192 and 223 it is a Class C address
224 and 239 it is a Class D address
240 and 255 it is a Class E address.

127 is reserved for loop back and is used for internal testing on the local machine.

Now we can see how the class of address determines which part belongs to the network (N) and which part belongs to the node (n).
Class A -- NNNNNNNN.nnnnnnnn.nnnnnnn.nnnnnnn
Class B -- NNNNNNNN.NNNNNNNN.nnnnnnnn.nnnnnnnn
Class C -- NNNNNNNN.NNNNNNNN.NNNNNNNN.nnnnnnnn

150.215.17.9 is a Class B address so its Network is defined by the first two octets and it's node is defined by the last 2 octets.
Class D addresses are reserved for multicasting and Class E addresses are reserved for future use so they should not be used.

In order to specify a network with an IP address the node section is set to all 0's. For example, 150.215.0.0 specifies the network for the above address. When the node section is set to all ones it specifies a broadcast that is sent to all hosts on a network. 150.215.255.255 specifies a broadcast address.

IP Subnet Addressing
There are several reasons why you subnet a network. You may want to incorporate different types of physical networks into your IP network. Implementing subnets helps to control network traffic. On an Ethernet network every machine on the same physical network sees all the packets of data sent out on the network. In heavy traffic situations this can result in collisions making the network performance painfully slow. In both these situations routers or gateways are used to segregate networks. The router therefore breaks the network into multiple subnets.

Subnet Masking
Applying a subnet mask to an address allows you to identify the network and node sections of an IP address. Doing a "Bitwise AND operation" on the IP address and the subnet mask results in the network address. For example:

10010110.11010111.00010001.00001001 150.215.017.009
11111111.11111111.00000000.00000000 255.255.000.000
-------------------------------------------------------------------
10010110.11010111.00000000.00000000 150.215.000.000

This result may seem familiar to you because Class A, B and C addresses have a self-encoded or default subnet mask built in.
Class A - 255.0.0.0
Class B - 255.255.0.0
Class C - 255.255.255.0

Or in binary form
Class A - 11111111.00000000.00000000.00000000
Class B - 11111111.11111111.00000000.00000000
Class C - 11111111.11111111.11111111.00000000

More Restrictive Subnet Masks
Additional bits can be added on to the subnet mask for a given class to further subnet a network. When a "Bitwise AND Operation" is performed on the subnet mask and IP address the result from the addition bits defines the subnet address. However there are some restrictions on the subnet address. Network addresses of all 0's and all 1's are reserved for specifying this network (when a host does not know its network address) and all hosts (broadcast address) respectively. This also applies to subnets. Therefore:
A subnet address cannot be all 0's or all 1's.
This also implies that a 1-bit subnet mask is not allowed.

Here is an example:
10010110.11010111.00010001.00001001 150.215.017.009 IP Address
11111111.11111111.11110000.00000000 255.255.240.000 Subnet Mask
----------------------------------------------------------------------------------------
10010110.11010111.00010000.00000000 150.215.016.000 Network address

In the previous example a 4-bit subnet mask was used. The subnet in this case was 1. There are 16 total, 14 usable subnets available with this mask (remember subnets with all 0's and all 1's are not allowed). Each subnet has 4,094 nodes (because of broadcast and network restrictions). This gives a total of 57,316 nodes for the entire class B address. Notice that this is less than the 65,534 nodes an unsubnetted class B address would have. Subnetting always reduces the number of possible nodes for a given network. Note that although subnet masks with non-contiguous mask bits are allowed, they are not recommended.

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What is Citrix?
Simply put, Citrix Metaframe allows you to run applications you have at work from anywhere in the world or on your local area network (LAN).

You have real time access to run your applications as fast as if you are in the office. It does not matter if you're using a dialup modem, DSL, ISDN, or T1 to access citrix remotely.

How does Citrix work?
We begin with Windows 2000 Server with Terminal Services Installed.

Then, we add Citrix Metaframe. This product allows multiple users to run multiple applications on the Citrix Server at the same time. When you run applications on the Citrix Server, the screen shots are sent to your computer and, in return, your keyboard input and mouse movements are sent to the Citrix Server.

How much bandwidth does it take?
The average connection uses 10K to 20K of bandwidth per connection. Hence, a 28K dial-up modem is enough to access a Citrix Server.

What kind of operating system can I use to access Citrix?
DOS, Windows 95, Windows 98, Windows CE (Thin Client), Windows NT 4.0,
Windows 2000, Windows XP, Unix, and Macintosh.

How fast of a computer do I need to access Citrix?
Virtually any personal computer can access Citrix (e.g., 386 to Pentium 4). In fact, it does not matter how fast your computer is, it will run with virtually the same speed on Citrix.

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