The next item in the text is this chapter's overview of internal hardware components. The picture on page 250 shows a lot of components, but it does not show where they actually fit in this exploded view of a computer. It does, however, present the reader with some idea about the inside of a computer: lots of components with cables. You should become familiar with this list of common components:
A discussion of the Central Processing Unit (CPU) begins on page 252. Features of CPUs that are used to rate and compare them:
CPU cooling fans are an important part of the computer. It should be remembered that air flow through a computer will be disrupted if openings are made in the case that are not intended to be open. A user might remove a plate on the back or side of a computer, intending to install an expansion card whose ports would be accessed through the new opening. If the user never installs the card and never replaces the cover plate, the new hole in the case will cause air to flow in unintended directions, ineffectively cooling (or not cooling) various components. Other optional cooling equipment:
The real world is sometimes digital, but more often analog. To understand the difference, consider the difference between a light with an on/off switch and a light with a dimmer switch. Think of the standard on/off switch as a digital switch, because it has only two states (on and off) and has to be in one state or the other. Think of the light with a dimmer switch as analog, because it has many possible states, ranging from off through dim to very bright. Data in a computer system is held in binary digits, ones and zeros. You can think of these digits as switches that are either on (holding a one) or off (holding a zero). Since people tend to shorten phrases that they use a lot, computer people shorten the phrase binary digit to bit.
Since a bit is not complex enough to represent more than two things (on and off), sequences of bits are used to represent characters. You are familiar with this idea already. How many things could we discuss if we only had two symbols to stand for them? We use 26 letters in English (and more or fewer letters in other languages) to make words. We use sequences to make more complex symbols.
Two code systems that use bit patterns (sequences) to represent data are: ASCII, which is used on most PCs, and EBCDIC, which has been used mostly on IBM mainframes. The two systems are different, but they serve the same purpose: characters are assigned unique bit patterns, and those bit patterns are used by a computer to represent the characters in memory. In most cases, a character is represented by eight bits. We refer to a sequence of eight bits as a byte. (More details in the added value section below.)
The chapter continues with a section about physical memory in a computer:
ROM chips are generally meant to hold instructions that do not change very often. Some of these chips hold the BIOS (Basic Input Output System) of the computer, which is not meant to change until the manufacturer makes an upgrade available. Data stored in ROM memory does not require power to be retained on the chip, but it does require power to use it or to reprogram it.
RAM comes in several sorts of packages. Some RAM chips used to be soldered on motherboards or on expansion cards. RAM that is meant to be replaced by the user is more often contained on a SIMM (Single Inline Memory Module) or a DIMM (Dual Inline Memory Module). SIMMs have chips on one side of a small card, while DIMMs typically have chips on both sides of the card. The text explains that DIMMs and SIMMs have contacts on both sides of their cards, but the ones on the back side of a SIMM are for different circuits.
SIMMs and DIMMs can be either DRAM (dynamic) or SRAM (static). The word static is misleading here. DRAM chips need to have their data refreshed dynamically every few milliseconds (a common value is every 4 milliseconds) with a jolt of power. SRAM chips do not need to have this periodic jolt, and will retain the data placed on them without the jolt, but only until the main power of the computer is turned. In this respect, DRAM and SRAM chips will both lose their data when the computer is turned off. SRAM chips are more expensive, but give faster access, so they are used where this speed advantage is necessary.
To check out options on buying RAM, I recommend looking at the Crucial Technology web site. Dr. Andrews recommends this site and the Kingston Technology site as well. As an exercise, assume you are buying memory for a Dell Optiplex GX1 (an older model) whose processor runs at 450 MHz. The computer currently has 128 MB of RAM. How much will it cost to take this system to the maximum RAM supported for it?
More on SIMMs, and DIMMs:
Adding and upgrading RAM are discussed. The author presents a list of questions that should be answered before buying RAM:
A critical issue brought up in this section is the idea of banks of memory. Typically, you have several slots in the motherboard into which you can insert SIMMs or DIMMs. Often, SIMM slots are grouped in banks. A bank could be one slot, two slots, four slots, or another combination. The short story is that if you use a bank, you must fill it. Also, if you use a bank you must use the same kind of SIMM in each slot in the bank. Different banks may have different size SIMMs in them, and may have different speed SIMMs in them. However, the RAM of the system will all run at the same speed as the slowest SIMM in the computer.
Installing memory is an important concept. You often must insert a SIMM module into the socket at an angle (45 degrees is preferred), then straighten it a bit to lock it into place. Sometimes with DIMMs there are actual locks to move at the ends of the socket, sometimes not. Inserting a DIMM straight down into its socket should engage the locks when the module is seated (pushed down all the way).
One place a speed advantage is useful is for cache memory. A cache is a place where data or instructions are placed that the computer expects to need soon or often. A separate controller chip was sometimes assigned the task of placing frequently used instructions in cache memory. In modern computers, the cache controller is more likely to be part of the main processor.
It was often necessary on older computers to adjust the CMOS settings after changing the amount of memory on the motherboard. Newer computers tend to sense the new amount automatically. If your system does not recognize the memory you have added, consider upgrading the BIOS.
Adding RAM can be tricky. If the system misbehaves after changing the installed RAM, check all the factors. Is it physically installed correctly? Does the BIOS recognize it? Did you buy the right kind? Have you put more memory on the motherboard than it can address? Any of these problems will lead to a computer that does not work.
An important concept from page 267: CMOS (Complementary Metal-Oxide Semiconductor) chips are meant to retain configuration settings for your computer while the power is turned off. This is usually accomplished by having a battery in the computer that supplies a small current to the CMOS chips. CMOS chips are used because they produce less heat than other types, and because they require less power to retain information stored in them.
Facts to remember about CMOS settings:
A short discussion about adapters begins on page 268. It mentions that there are several kinds of adapter cards that can be installed in a regular desktop computer to add functionality such as better sound or video, network ports, or MIDI ports for a musical device. It ignores the fact that you must match the card you buy to the kind of slot you have available in the computer. Failing to do this will waste your time, your money, and possibly your computer. The text tells you that it is necessary to power down a computer before plugging in a new adapter card, and that this is not necessary with a USB device. It does not mention that it is possible to ruin the USB device if you do not safely release it before pulling it out of the USB socket. See the article behind that link. Pull them out hot if you want to, but you have been warned.
A discussion of buses begins on page 270. It is misleading in that it discusses only data buses, which I have already discussed, and ignores the concept of adapter type buses. Be aware that there have been many slot-bus types over the years, and no doubt there will be more. Don't assume that the adapter you buy will fit the slot you want it to fit without checking. Follow this link to an Internet page with illustrations of several adapter bus types.
Since the book is not covering several topics that we are supposed to cover, let's look at a few facts about IP addresses, subnets, and DHCP.
||Position: Tens||Position: Ones
|Value of digit times value of position:300
||Value of digit times value of position:20||Value of digit times value of position:1|
In a way, binary notation is easier. There are only two digits: 1 and 0. The value of a binary number is the sum of the values of the positions that hold a 1. Let's look at how that works.
The largest number that can be stored in one byte is 255. This is the sum of the values of all eight positions in the byte. Converting a binary number to decimal is simple: add the values of all the positions that hold a one. The only trick is to know the value of each position. They are all powers of two. Start on the right with 1, and double the value to get the value of the next position to the left: 2, 4, 8, 16, 32, 64, and 128. If that isn't working for you, listen to Danny Kaye sing, and pay attention to the children in the background. Or if that's too old, try Paul McCartney.
|Value of Position (if a 1 is in it):||128||64||32||16||8||4||2||1|
How do you convert a decimal number (less than 256) to binary? (For bigger numbers, you need more than one byte.)
You do a series of subtraction problems, one for each position in the byte, starting from the left. For example, let's convert 175 to binary.
|Conversion of 175 (above)||1||0||1||0||1||1||1||1|
Knowing binary helps us understand the five address classes in an IP network. The first three classes can be described by the number of bytes assigned to the network portion of their addresses:
Class D and E addresses use portions of the fourth byte as well for network addressing. You may wish to know that only class A, B, and C addresses are in general use. Class D addresses are for multicasting (messages to groups of machines), and class E addresses are for experimental use.
One way to recognize the class of a given address is to memorize the range of possible addresses in each class, shown in the graphic on this page. That's what I had to do when I learned it. There is a better way.
The five classes of addresses are defined as limited to specific ranges of values of the first byte in the address. The numeric ranges are hard to remember until you see a chart that explains what the ranges have to do with the binary version of the first octet.
|Value of Position (if a 1 is in it):||128||64||32||16||8||4||2||1|
Reading from left to right, if the first bit (position 7, above) of the first octet is a zero, that octet must represent a number less than 128. (Why? Because all the other position values only add up to 127.) This defines a class A address: the first byte must be 127 or less. Consider it this way:
So, if you can convert the first byte of an address to binary notation, you can tell the address class by the position of the first 0 in it, reading from left to right.
Consider the task of assigning addresses to hosts within your network. Host addresses should follow the network address scheme you are assigned. If your network address was 184.108.40.206, you would have a class B address, and all host IP addresses should begin with 150.60.
So, how many can you have? Let's consider theory for a moment. In a perfect world, assume you are assigned address 220.127.116.11, which is a class A network address. (No, you can't have it, it's taken, but just think about it.) How many hosts can exist on that network? Your first byte (class A) determines your network in this case, so we can assign numbers to hosts with the other three bytes. Each byte can hold up to 256 different numbers. Three bytes means 256 x 256 x 256 which takes us to 16,777,216 different addresses on that network. Sort of. (This is why class A addresses are assigned to ISPs.)
You can't actually assign the address where all host bits are set to 0, because that is the address of the network itself, and it needs it. Another rule (that is almost always true) is that you can't assign the address where all host bits are set to 1, because that is the broadcast address for the network, used for messages sent to all hosts. If you need to subdivide this network into subnets, each one will also need its own network address and broadcast address. We will come back to this idea.
Say you just bought a router for your home. If you do not have an assigned address, you could use any address scheme you wanted as long as you did not attach to the Internet (or any other network). In reality, everyone wants or needs the Internet, so you should use a private address scheme, as shown in the chart below. You can choose any address scheme for your network that gives you enough host IDs. The proper use of private addresses tells any network staff who see them that these are not registered public addresses, and these machines need "help" to get to the Internet. (That's foreshadowing: the plot thickens.)
Private addresses work inside a network, but their traffic cannot be routed to other networks outside that one. (Great, then how do I get to the Internet? More magic in a minute.) To understand this, you need to know that every IP address in the world was meant to be unique.
Wait a minute, if there are about 128 class A networks, with not quite 17 million hosts on each one, then how do we get the whole world on the Internet? The short answer is, there is more to it. IPv6 addresses are one way, but we have a huge number of networks using only IPv4. Keep reading.
Addresses that are visible to the world are called registered or public addresses. This scheme would allow any IP addressed machine to contact any other (in theory) because the address would identify the network and the host uniquely. (It was also believed that there would be networks that would have no need to contact other networks. Yeah, right. I never need to check my mail, buy on Amazon, download a driver, post web pages for students,...) At a certain point, the Internet became valuable to everyone, and the world began to run out of addresses.
So the Internet Assigned Numbers Authority (IANA) designated some address ranges as private or unregistered addresses. They are also called nonroutable addresses, although that is not exactly an accurate name. They are never used on the Internet as real addresses, which is the point.
Any address beginning with a 10, for example, is assumed to be a private address. This is the format used in many networks, because it provides lots of addresses for hosts, and lots of room for subnets. A subnet is a subdivision of your network. You will need this in a few paragraphs.
Within any organization, addresses in these private ranges may be used without registering the addresses with IANA or ICANN, the Internet Corporation for Assigned Names and Numbers. Each address you use within your network must still be unique in your network. The problem is that there is no guarantee whatsoever that any address I use in my organization is not already in use in your organization, which makes direct networking between our networks unreliable, if not impossible.
The magic part: To access the Internet, traffic from a private address network passes through a router that acts as a proxy server, providing a shared connection with a registered address. That router has both a private address on your network, and a registered public address that lets it communicate with other networks. The proxy server shares its public address with the devices on your network, allowing them to send signals to other networks through it. The service that does this sharing of the address is called Network Address Translation (NAT).
This is the same principle that allows a home user to set up a router, connect computers to it, and connect the router to a cable system modem. Comcast/Charter/Frontier/whoever assigns an address to the cable modem, which is given to your router. Then the router assigns private addresses to all the devices on your network, and everyone can get to the Internet.
Now, let's consider subnets. Suppose your company has six divisions or locations, and logically needs six networks. Logically, you would want to be assigned six different network addresses. Suppose, however, that you either cannot be assigned or cannot afford licenses for six networks. Then you take the one network address that you can obtain and create six subnets. (Six, by the way, is not a magic number, it is just an example. Maybe you need a hundred. Same principle applies.)
Subnetting works by borrowing bits from the host portion of an address, and using those bits to identify subsections of your network. (By the way, borrowing is the word that is always used, but it is more like stealing. We have no intention of giving them back.)
The use of borrowed bits only works because of subnet masks. A subnet mask is like a pattern or filter that a host is told about when it is set up to use a network. A subnet mask tells hosts on a network which bits in an address are network address bits and which bits are host address bits. It does it by the use of 1s and 0s. Consider the table below:
||Decimal Mask||Binary Mask|
Network devices read a mask to learn how to interpret addresses. A mask is always a series of 1s followed by a series of 0s. Address positions marked by 1s in a mask are considered network address positions. Address positions marked by 0s in a mask are considered host address positions.
Another way of saying this is that certain address bits are considered to be network address bits and the rest are considered host address bits. The actual method used involves Boolean math, but understanding it is not critical to understanding or using the concept. When a device reads an actual IP address, the rule from the subnet mask is applied, and the device understands which bits are the net address and which are the host address.
By borrowing one bit from the host address bits, two subnets are theoretically
possible. However, as a general rule, subnet numbers using all 1s and
all 0s are not used (for the reasons noted above), so borrowing one bit will probably not yield any
usable subnet addresses. This is why the formula for number of usable
2N - 2 = number of subnets (where N is the number of bits borrowed).
Assume we borrow two bits in each of the three classes above. Borrowed bits are shown in red in the resulting subnet mask numbers below:
|Decimal Mask||Binary Mask|
Note that the subnet masks above do not match the standard masks from the previous table. The standard masks are classful masks, because they match the intended use of class address schemes. The masks above are classless, because they do not match any network class. Those words do not mean they are less functional, they just tell networking staff what you have done.
Be aware that routers on the Internet only use the network bits of an address for routing. Their job is only to find the right network. Routers connecting subnets within a network must use the network, subnet, and host bits for routing.
Rules about addresses when using subnets:
As stated above, in an IP network each device needs a unique IP address. Assignment of these addresses takes some planning. First, there are two major approaches:
Dynamic Host Configuration Protocol (DHCP) service allows us to dynamically assign IP addresses to hosts on an IP network. Three methods for assigning addresses:
Whether you use dynamic or static assignment, you can assign more than the IP address of a device with DHCP. You can also specify what the subnet mask for your device is, what server to use for Domain Name Service (DNS), and what server to use for Gateway service (routes to other networks). A DHCP server can provide this information when the device powers up, or it can be configured by hand, if you have the rest of forever to get your work done. This should give you a little more respect for the job your wired or wireless router is doing at home.
Assignment 2: Read the material above about subnetting. We will do some exercises about this subject next week.