CIS 1110A - Computer Operating Systems and Maintenance


Module 8

This chapter continues the lesson on networking with a discussion of infrastructure. Objectives important to this lesson:

  1. TCP/IP protocols in Windows
  2. Network hardware
  3. Setting up and troubleshooting a LAN
  4. Troubleshooting
  5. Current assignments

Concepts:

This is a 50 page chapter with a two page summary. I will try to reach a compromise with those two facts. The chapter begins with a metaphor about networks:

  • they are constructed in layers
  • they operate by processes in one layer passing data to other processes on other layers
  • each process may do something with the data, or may simply make it understandable to the next process to get it

There are several network models. The one most commonly used was created by the International Organization for Standardization, now called ISO. (No it isn't an acronym, it is from the Greek word isos, meaning equal.) Their model is called the Open Systems Interconnect (OSI) Reference Model, so it is also called the ISO-OSI model.

I am starting with this model because it is the one you will run into the most, and it makes several things clear that others do not. Once you understand this model, you will have a general, powerful reference for examining and comparing networks. There is a lot more to it than I will tell you today, so just take a moment to breathe and we will dive in.

The seven layers of the model are usually written in a list, numbering the top as layer seven and the bottom as layer one.

Layer Number ISO Layer Functional Description
7 Application services and programs
6 Presentation translation across networks
5 Session setting up and ending connections
4 Transport guaranteed delivery
3 Network find other networks; host and network addresses
2 Data-Link media access; MAC addresses
1 Physical wiring, bit transmission, sending and receiving network signals

Several mnemonic sentences exist to help us remember the proper order. I recommend "Please Do Not Throw Sausage Pizza Away", because this is in the correct numeric order (bottom to top, 1 to 7). If you want one that goes from top to bottom, try "All People Studying This Need Drastic Psychotherapy". On any certification test that covers this model, you MUST remember the correct order, the correct numbers, and the correct details for each layer.

The processes that happen in each layer usually communicate with processes in the next layer. Which way is next, up or down? It depends whether data is being passed out of the stack (down) or into it (up). Typically, a computer generates a request when an application (program) wants something that is on the network. The request starts at the top layer, and works down. The request is passed across the network (probably to a server) and the received request is passed up the layers on that computer. When a response is generated, the process reverses, going from layer 7, down to layer 1, and across the network. If that's confusing, play one of the videos below.



Our author uses a related but simpler network model to introduce the chapter: the TCP/IP model. It only has four layers, but they match the ISO-OSI model nicely. Consider the table below. The four layers of the TCP/IP model are shown next to the matching layers of the OSI model.

TCP/IP name OSI Layer name
Application layer Application
(layer 7)
Presentation
(layer 6)
Session
(layer 5)
Transport layer Transport
(layer 4)

Internet layer
Network
(layer 3)
Link layer Data Link
(layer 2)
Physical
(layer 1)

Dr. Andrews uses the TCP/IP model to explain where the network layers are engaged in a web page request. Imagine the diagram above as the request for a web page is being sent as a signal onto the Internet:

  • As the request is prepared to go, it starts at the Application layer, where the message is packaged by Application layer rules, then passed down to the Presentation layer. In the TCP/IP model, this is still part of the Application layer.
    • The Presentation layer receives the message, repackages it as needed by its rules, changes file system and character sets as needed, keeping the information from the Application layer inside the packets it makes, then hands its packets off to the Session layer.
    • The Session layer negotiates a connection with the next machine it needs to send to, which it does while it takes the received Presentation packets and repackages them as Session packets. These are handed off to the Transport layer.
  • The Transport layer continues the pattern: add your magic, wrap it around the received packets, and put them all in your own message units called segments. Segments are different sizes on different networks: what happens is like taking full gallons of milk and putting it in pint sized bottles, keeping in mind that they will be poured back into matching gallon bottles on the far end. This layer also identifies every segment, relates it in position to the ones that come before it and after it, and checks to make sure each delivery is received by the destination device. The segments are handed off to the Network layer, which is called the Internet layer in the TCP/IP model.
  • The Internet layer finds a routes that the request can take to get to the intended destination, adds IP addresses for source and destination, rewraps the segments as datagrams, and hands them to the Link layer.
  • The Link layer, which contains the ISO-OSI Data Link and Physical layers, does not change what is in the datagrams, but it adds MAC addresses for source and destination. The datagrams are re-wrapped as frames, and they are pushed to a network on the Physical layer.
    • The Physical layer takes the frames, which are perceived as a stream of bits, moves them as needed to a NIC, to the network medium, and to the next device, again and again, until the stream is processed by a NIC on a receiving machine, which may be the final destination (if the requester and the server are on the same network) or a router that passes the request to another network.

   
As noted above, the request is received by the destination machine and the process is conducted in reverse, unwrapping, recombining, and processing the request. The author points out that the operating system of each device spans the Application, Transport and Internet layers. That is because the network software is either part of a device's OS, or becomes part of it when the "client software" for a network is installed on the device.

The author turns to IP addressing on page 385 with a discussion of IPv4 addresses. The problem with this kind of lesson is that there is a lot of background to know. Sip your choice of soft drink and follow along.

Since the TCP/IP protocol stack was invented with networking in mind, IP addresses contain two parts: one to identify the address of the network a host is on, and the other part to identify the host itself. Every network is assigned an IPv4 address which could be one, two, or three bytes, depending on the class of the network (A, B, or C). The remaining byte or bytes are typically used for hosts on networks. (There are other ways to do it as well. Variations will be discussed shortly.)

Each byte in an IP address will be a number in the range 0 through 255, expressed in base 10 (decimal notation). You will need to be able to convert decimal notation to binary notation and vice versa, by hand. No calculator. A method of doing so is presented below.

First, some decimal math, for reference. Numbers are written in a positional notation: the value of a digit is determined by its position. For example, consider the number 56,211,324:

Values of Positions in a Byte
Value of Position: 10,000,000 1,000,000 100,000 10,000 1,000 100 10 1
Digits in example: 5 6 2 1 1 3 2 4
Value of Position times value of digit: 50,000,000 6,000,000 200,000 10,000 1,000 300 20 4

 

Some basic training in binary notation: a byte has eight bits. Each bit can be a digit in a binary number. Since we can only use 1s and 0s in binary notation, we either have (1) or don't have (0) the number of units represented by a position in the binary number.

Values of Positions in a Byte
Bit position: 7 6 5 4 3 2 1 0
Value of Position (if a 1 is in it): 128 64 32 16 8 4 2 1

To translate a binary number to decimal, just add the values of each position in which there is a 1. For example, 11010011 in binary. Begin to convert to decimal by making the value chart for binary. The chart is easy to make: start with 1, and work to the left. The next place is 2. Each position is worth twice the value of the position to its right. Don't count by twos, multiply each time by two.

Values of Positions in a Binary number
Value of each position: 128 64 32 16 8 4 2 1
Digits in example: 1 1 0 1 0 0 1 1

So, add 128 + 64 + 16 + 2 + 1 = 211. (Don't add the values of positions holding a zero.)

To convert from decimal to binary, try this. Start with a decimal number: 156. Compare it to the binary position table. Work from right to left like this. Can you subtract 128 (first position value) from the number (156)? Yes. Write down a 1 in the first position, and subtract.
156 - 128 = 28

Values of Positions in a Binary number
Value of each position: 128 64 32 16 8 4 2 1
Record digits: 1              

Continue across the table. Can you subtract 64 from 28? No. Write down a 0. Can you subtract 32 from 28? No. Write down a 0.

Values of Positions in a Binary number
Value of each position: 128 64 32 16 8 4 2 1
Record digits: 1 0 0          

Can you subtract 16 from 28? Yes. Write down a 1, and do the math. 28 - 16 = 12.

Values of Positions in a Binary number
Value of each position: 128 64 32 16 8 4 2 1
Record digits: 1 0 0 1        

Can you subtract 8 from 12? Yes. Write down a 1, and do the math. 12 - 8 = 4. 4 happens to be the value of the next position, so write down a 1, and fill the rest of the positions with 0s.

Values of Positions in a Binary number
Value of each position: 128 64 32 16 8 4 2 1
Record digits: 1 0 0 1 1 1 0 0

So, 156 decimal is 10011100 in binary. If you are working on a decimal number less than 256, an 8 position binary table is big enough. Larger numbers require a wider table.

There are five IP address classes you need to know. The first three classes can be described by the number of bytes assigned to the network portion of their addresses:

  • one byte for class A
  • two bytes for class B
  • three bytes for class C

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, and class E addresses are for experimental use.

You can recognize the class of an address by memorizing the range of values that occur in the leftmost byte of addresses in that class.

  • Class A - range is 0 to 127
  • Class B - range is 128 to 191
  • Class C- range is 192 to 223
  • Class D - range is 224 to 239
  • Class E - range is 240 to 255

You can do it that way, but it is painful. A better way to recognize the class of a given address is shown on page 7-5. The five classes of addresses are defined as limited to specific ranges within the first byte. The numeric ranges are hard to remember until you see a chart like the one on page 7-5 that explains the ranges have to do with the binary version of the first byte.

Reading from left to right, if the first bit (position 7, above) of the first byte is a zero, that byte must represent a number less than 128. This defines a class A address: the first byte must be 127 or less. Consider it this way:

  • Class A - first bit is a 0, range is 0 to 127
  • Class B - first bit is a 1, second is a 0, range is 128 to 191
  • Class C- first two bits are 1s, third is a 0, range is 192 to 223
  • Class D - first three bits are 1s, fourth is a 0, range is 224 to 239
  • Class E - first four bits are 1s, fifth is a 0, range is 240 to 255

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.

Page 7-8 has a list of some specific addresses that have special meanings:

  • 0.0.0.0 - the default route
  • 127.0.0.0 - reserved for loopback. 127.0.0.1 is defined as the Local Host. It means the computer you are on.
  • All network bits set to 0 - this would mean that the host bits are set to something other than 0, and that we mean that specific host on the current network.
  • All host bits set to 0 - this is the address for the network itself
  • Network bits or host bits all set to 1 - this is the equivalent of using wildcards, meaning all nets or all hosts
  • 255.255.255.255 - this is all wildcards, but refers to all hosts on this network

There is a general rule that you can't set the host bits of an address to all 1s or all 0s.The same is true for the network bits.

If you do not have an assigned network address, you could use any address scheme you wanted as long as you did not attach to the Internet. However, in reality, everyone wants or needs the Internet, so you should use a private address scheme, as shown in the chart on page 7-10.

Class Beginning Address Ending Address
Class A 10.0.0.0 10.255.255.255
Class B 172.16.0.0 172.31.255.255
Class C 192.168.0.0 192.168.255.255

Any address beginning with a 10, for example, is assumed to be a private address. This is the format used in a network I have some experience with. To access the Internet, traffic passes through a server that acts as a proxy, providing a shared connection with a registered address. That is, the proxy server has an actual registered Internet IP address, as well as a ten-dot address on our network. Our PCs send signals to the proxy server, which shares its connection to the Internet. It assigns a separate port number to each PC on our network, and marks our signals to the Internet with that port number. This makes it possible to track the senders and receivers of each incoming and outgoing message, while still using only one registered IP address.

The process the proxy server uses to convert addresses on our ten-dot network to addresses usable on the Internet is an example of Network Address Translation (NAT).

Private addressing is often done along with subnetting. Suppose your company has six divisions or locations, and logically needs six networks. Logically, you would want to be assigned six different network addresses (such as 132.132.0.0). Suppose, however, that you either cannot get or cannot afford licenses for six networks. Then you take the one network address that you do have and create six subnets. (Six, by the way, is not a magic number, it is just an example.)

Reasons for creating subnets:

  • To extend the network - this reason applies if you have reached a media or node limit, and still have network address space to use
  • To reduce congestion - this reason is similar to the reason you use bridges and switches: to increase the number of collision domains. It works best if most traffic can be contained within single networks.
  • To reduce CPU use - because all hosts must listen to broadcast traffic, to determine if it is for them, a greater load is placed on each host in larger networks. Multiple smaller networks have fewer broadcasts.
  • To isolate network problems - if you have multiple networks, any problem affecting one will be less likely to affect the others.
  • To improve security - if you have sensitive transmissions, keeping them on one of several networks will limit the opportunity to eavesdrop
  • To use multiple media - it is easier to have multiple media types if they are on different subnets. Routers will readily interconnect the subnets.

Subnetting works by borrowing bits from the host portion of an address, and using those bits to identify subsections of your network. The use of borrowed bits only works because of subnet masks. 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. (What else have we got?) Consider the table below:

Subnet Masks for Classes A, B, and C

Decimal Mask Binary Mask
Class A 255.0.0.0 11111111.00000000.00000000.00000000
Class B 255.255.0.0 11111111.11111111.00000000.00000000
Class C 255.255.255.0 11111111.11111111.11111111.00000000

Network devices read a mask to learn how to interpret addresses. 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 for what address.

The following table shows the subnet masks that result when borrowing 1, 2, and 3 bits of the host address. Note the first example: by borrowing one bit, two subnets are theoretically possible. That is, the subnet bit could be set to 1 or 0: two possible subnets. However, as a general rule, subnet numbers using all 1s and all 0s are not used, so borrowing one bit will probably not yield any usable subnet addresses. This is why the formula for number of subnets is :
2N - 2 = number of subnets (where N is the number of bits borrowed)
You will want to keep in mind that some network operating systems do allow the use of all 1s and/or all 0s. Assume we borrow two bits in each of the three cases above:

Subnet Masks if Borrowing 2 Bits

Decimal Mask Binary Mask
Class A 255.192.0.0 11111111.11000000.00000000.00000000
Class B 255.255.192.0 11111111.11111111.11000000.00000000
Class C 255.255.255.192 11111111.11111111.11111111.11000000

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. Look at the subnet masks in binary. Even when a network is using subnetting, all subnet masks are a series of 1s followed by a series of 0s. The series of 1s always tells you which positions in addresses are network positions, and the series of 0s always tells you which positions in addresses are host positions. In the table above, I have marked some positions in red, indicating which bits mark the actual subnetworks. They are important in planning and managing subnets.

Be aware that routers on the Internet only use the network bits of an address for routing. Routers connecting subnets within a network must use the network, subnet, and host bits for routing. Host addresses may be reused from one subnet to the next, but not within a subnet.

Consider class C networks. Obviously, you cannot use eight bits to define a subnet on a class C network: you only have eight bits to define a host address to begin with, and you must use some of them for the host address. You must strike a balance between how many subnets you need and how many hosts you may put on each subnet. Suppose an administrator has decided to borrow 3 bits from the host byte for subnets, leaving 5 bits for host addresses. To calculate how many usable subnets are obtained when borrowing a specific number of bits, use the formula above:
2N - 2 = number of subnets (where N is the number of bits borrowed).
Do not subtract the 2 if you are able to use all 1s and all 0s in the subnet address, which some network equipment allows.

To calculate the number of hosts possible for each subnet, do the same calculation, except that for the value of N, you use the TOTAL number of host bits available in the address. For instance, if this were a class B address, and you were borrowing 3 bits from the third byte for subnet addresses, the remaining 5 bits in the third byte and the 8 bits in the fourth byte would give you 13 as the value of N. Some methods call the exponent M when calculating the possible hosts, but the math is the same.

The author mentions that counting the number of network address bits is useful, but it is easy to make a mistake, A better way of observing the number of network bits is to use Classless Inter-Domain Routing (CIDR). CIDR is a router standard that simplifies the use of classless subnet masks. In the CIDR standard, IP addresses are followed by a slash and the decimal number of bits used in the network portion of the subnet mask.

An example of CIDR notation might be 220.12.78.0/24. This example shows a network address, 220.12.78.0. It is followed by a forward slash, and the number 24. The number 24 means that the subnet mask for addresses on this network uses 24 bits as network identifiers. This is the same thing as saying that the subnet mask for this network uses 3 bytes as network identifiers, which is the same thing as saying the subnet mask is 255.255.255.0. In the table below, the same network is described three ways. Each is telling us the same thing: IP addresses on this network hold network information in the first three bytes, and host information in the fourth.

3 descriptions of the same network Example
subnet mask 255.255.255.0
number of bytes that hold network bits 3
CIDR notation /24

When addresses are sent on networks that allow CIDR notation, they can include this helpful notation that is the equivalent of sending the subnet mask along with the address. This is particularly helpful in networks that use subnetting. In such cases, the number following the forward slash probably will not be a multiple of 8.

When I first started working in IT, the IP addresses used by equipment in my agency were often registered/global IP addresses, unique addresses that were assigned by a license from IANA. IP address ranges in the various classes are assigned by the Internet Assigned Numbers Authority (IANA). Ranges of addresses assigned within a given class are called class licenses. Once upon a time, this was done by one person. Now, IANA doles out licenses to Regional Internet Registries (RIRs), who assign them to Internet Service Providers (ISPs).

Most homes have no need of public facing addresses, and they are better off without them. Instead, they use private addresses, ranges that are reserved for networks that do not connect to the Internet. Say, what? Yeah, that didn't last long. If you are using private addressing in your home or business, you will also use Network Address Translation (NAT) through your ISP. You essentially borrow one of the ISP's registered addresses to send and receive traffic across the Internet. Here is a video to make it easier to understand:


Dr. Andrews presents a couple of pages on IPv6 addresses, which is more of an accomplishment than you might think. Explaining it in such a small space is remarkable.

  • IPv6 addresses have 128 bits, which are written as eight blocks of hexadecimal digits (base 16), each block being four hex characters; blocks are separated by colons when written
  • shortcut: if a block has leading 0s, you can leave those out, since the numbers would be the same (e.g. 00B4 = B4)
  • more complicated shortcut: if a block is all 0s, you can just leave out the 0s, write a colon, and get on with the next block; this results in a double colon (2244:0000:3535 could just be 2244::3535)
  • Take a deep breath: if there are two blocks in the address that are all 0s, you can only use the "double colon" notation for one of them (this was not a rule in the first version)

Dr. Andrews gives us some terms that are used in IPv6 addressing (and some used in IPv4 as well):

  • link - a LAN or a WAN
  • node - any device that connects to the network
  • interface ID - the last four blocks in an IPv6 address; identifies the local network device NIC; you can have multiple addresses if you have multiple NICs
    You take the MAC address of the NIC, insert FFFE in the middle, and invert the seventh bit. This generates the 64 bit interface ID.
    Example: MAC address is 16 F8 D8 EF A7 53
        Insert FFFE after the third pair: 16F8D8 FFFE EFA753
        The first pair is 16, which is 0001 0110 in binary, so the 7th bit becomes 0, which gives us 0001 0100 which is 14.
        So, 14F8:D8FF:FEEF:A753 becomes the interface ID for that NIC. That's half of he IPv6 address.
    (Note: my IPv6 address does not follow this formula. It just has four hex characters inserted in front of the MAC address.)
  • neighbors - nodes on the same network
  • multicast - a transmission being sent to multiple node who are all listening for a specific address
  • anycast - a transmission meant to identify the closest node that can answer a request; routers use this to find the closest router that knows a route to a destination
  • unicast - a transmission that is sent to a specific address (as most are)
  • there are some other terms on page 390 which you may want to learn

Our friend, Linus Sebastian, would like to talk about IPv6 so let's give him a shot. This video discusses IP addressing, and touches on NAT, too.


The text explains that DNS is a system that takes advantage of the fact that IP addresses are hard to remember, but domain names are easier. If we assume that a network can be given a domain name, and a computer can be given a hostname, then it makes sense that we used to have a server called crux.baker.edu. Machine called crux, in the sub-domain baker, in the edu domain. This is an example of a fully qualified domain name. It works on the DNS system that everyone uses.

The text also wants you to know the major differences between UDP and TCP, the two protocols used for delivery of most transmissions. The TCP protocol operates on the Transport layer, which makes this layer associated with the word reliable. If a packet is lost or received in a damaged state, a replacement copy of the packet is requested. This is one aspect of reliable, guaranteed delivery.

Just to make things complicated, the Transport layer is also where the User Datagram Protocol (UDP) lives. If TCP is like sending a registered letter to a recipient (we know it is going to get there), UDP is like sending a postcard with our best guess at an address. UDP is quick and easy, but it may not get the job done. TCP is sometimes called a connection-oriented service. UDP is called a connectionless service. Quick tip: If you need to remember whether a protocol is connection-oriented or connectionless, look at the first letter of the protocol name. If it is a consonant, it is connection-oriented. If it is a vowel, it is connectionless. This is only true for protocols for which we care about this concept. Taking Care of Packets? Uncaring Delivery of Packets? ("Bloody American style shooting...")

The text offers a pair of related lists. First there is a list of protocols commonly used in the TCP/IP suite for specific purposes. Here are several examples,

Application Layer Protocols (upper 3 layers of the OSI model)
  • Telnet - a protocol for connecting to a different computer, and making your workstation a terminal to that other computer
  • File Transfer Protocol (FTP) - allows users to copy files as though using local devices. It supports the use of user IDs and passwords.
  • Trivial File Transfer Protocol (TFTP) - also allows users to copy files, but does not support User IDs and passwords
  • Hypertext Transfer Protocol (HTTP) - the file transfer protocol used on the World Wide Web
  • Network File System (NFS) - this protocol allows users on one operating system use files on a device running a different operating system
  • Simple Mail Transfer Protocol (SMTP) - this is the standard e-mail transport protocol for TCP/IP stacks. It depends on TCP for message routing. The text points out that SMTP is used for sending e-mail, and Post Office Protocol 3 (POP3) is used on the same systems for retrieving it.
  • Line Printer Daemon (LPD) - this protocol receives print jobs and sends them to network printers using TCP/IP
  • X Window - X Window is a GUI client interface for running programs on a UNIX server
  • Simple Network Management Protocol (SNMP) - a basic network management tool, it generates a lot of traffic about the performance of the network.
  • Domain Name System (DNS) - discussed in a previous chapter, converts a registered domain name to an IP address
  • Windows Internet Naming Service (WINS) - works like DNS, but it takes NetBIOS names used on Microsoft networks, and converts them to IP addresses
  • Dynamic Host Configuration Protocol (DHCP) - DHCP provides not only the IP address, but also configuration settings for the host. DHCP requests are broadcast requests. Broadcast requests are not forwarded by routers, so a DHCP server must be on the same network segment as the device making the request. (Unless we decide to forward those requests, as is often done.)
  • Bootstrap Protocol (BootP) - BootP is a simpler protocol than DHCP. A device on a network sends its MAC address along with a request for an IP address. A BootP server can only assign an IP address to that device if its MAC address is stored in a BootP table on the server, along with an IP address that is meant for the device. (A DHCP server can service devices dynamically, as long as it has more IP addresses available to assign.)
Transport layer protocols
  • TCP - Transmission Control Protocol provides reliable, connection-oriented delivery service. It creates virtual circuits, which are logically similar to the circuits created for telephone connections. It creates data packages called segments, which include source and destination port numbers, segment numbers, acknowledgement numbers, and checksum information. Your text, of course, does not define what a port number is at this time. Think of a port number as a pointer to a location in the working memory of a device. It stands for the memory address of the program or service you are contacting. Use TCP with services that require guaranteed delivery and proper sequencing of segments, such as Voice over IP.
  • UDP - User Datagram Protocol creates segments as well, but they contain only addressing, length, and checksum information in their headers. No numbering, no acknowledgements, no guarantee of delivery. This type of protocol is called connectionless. Compared to TCP it is a thin or light protocol. UDP should be used in conjunction with other protocols that contain their own connection services or that do not need this service. UDP is used with SNMP because SNMP creates a great deal of traffic, and TCP would cause this traffic to consume too much bandwidth. UDP is used with NFS because NFS has its own connection-oriented aspects and does not need TCP to provide this feature.

As noted above, a simple test for whether a protocol under consideration is connection oriented or connectionless: if you are asked this question about a protocol, does its name start with a consonant or a vowel? UDP, IPX, and IP are connectionless. TCP, SPX, and NFS are connection-oriented.

The text discusses port numbers, telling us that numbers below 1024 are called well-known port numbers. A port number can be any number from 1 through 65535. Several port numbers are assigned to specific services through conventions established by ICANN. (A listing may be found in RFC 1700.) If you follow the link above to the list of ports used by specific services, you will find more information than is in your text. You will also find that although the text says that NNTP uses UDP, it also uses TCP.

Port Service Service works with...
20 FTP, data TCP
21 FTP, control TCP
23 Telnet TCP
25 SMTP TCP
53 DNS TCP, UDP
69 TFTP UDP
80 HTTP TCP
110 POP3 UDP
119 NNTP (Network News protocol) TCP, UDP
161 SNMP UDP
443 HTTPS TCP

Some references say ports 1024 through 65535 are Registered ports. Others say that 1024 through 49151 are the Registered ports, and that 49152 through 65535 are Dynamic ports.

The text mentions switches and hubs. You don't want a hub, and you probably won't find one in a store. Hubs have multiple ports (usually RJ-45 jacks). Any signal coming into a hub comes back out on all ports. Bad use of bandwidth. Now that you have that, Let's hear Linus on switches, WAPs, modems, and routers. You can stop watching after he explains hubs, unless you want to see a three year old minute and a half commercial.


The text presents about a page of types of servers you might find on a network. Be aware that the word "server" does not mean a computer. It means a service available on a network. All of the servers in Dr. Andrews' list could be running on one box. That's an extreme idea, but it could be done if one server-box is all you had.

There is brief discussion of network security devices, but it is not very educational. There is a short discussion of Ethernet cables with some terrible pictures. There are a few more filler concepts. We have already discussed the useful ones.

The last section of the chapter is about troubleshooting. Run through this section.

  • A rule that applies to the flow chart on page 421 is to work outward from the center in stages. Can you ping your own device? Can you ping another your router? Can you ping an address on the Internet? Where does this sequence fail?
  • What connections can you check? What devices can you restart? Are there trouble lights flashing? Are expected operating lights off? Is the thing on fire?



Assignments

  1. Read the chapter, and the next one for next week.
  2. Complete the assignments and class discussion made in this module, which are due by 6pm next week.