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Trouble Spots

Trouble Spots

As with any exam, it will vary from person to person what is deemed to be difficult. Since IPv6 is a new configuration being implemented to replace IPv4, it may cause you more trouble than other subjects. Subnetting is another item that is sometimes difficult for people to wrap their minds around. I hope to explain issues dealing with subnetting, the OSI model, IPv4 and IPv6 below.

Internet Protocol version 4

Certain IP address are private and not routable over the Internet:

  • 10.0.0.0 – 10.255.255.255
  • 172.16.0.0 – 172.31.255.255
  • 192.168.0.0 – 192.168.255.255

Subnetting is the process of dividing the total available IP addresses for a network into subnetworks, or subnets. On the exam, you may have several questions about subnetting, so you will have to understand it. You will need to be able to calculate them quickly in order to save time. There are many methods for figuring subnets and hosts. One of the simplest and quickest ways is to memorize the table below. It is not as hard to memorize as it looks, because all you have to do is double the number and two in order to get the next one above it.

Table 1 Subnetting and Hosts

Subnet Mask

Subnets

# Class A Hosts

# Class B Hosts

# Class C Hosts

192

2

4,194,302

16,382

62

224

6

2,097,150

8,190

30

240

14

1,048,574

4,094

14

248

30

524,286

2,046

6

252

62

262,142

1,022

2

254

126

131,070

510

NA

255

254

65,534

254

NA

Internet Protocol version 6 (IPv6)

The IPv6 128-bit address is divided along 16-bit boundaries. Each 16-bit block is then converted to a 4-digit hexadecimal number, separated by colons. The resulting representation is called colon-hexadecimal. This is in contrast to the 32-bit IPv4 address represented in dotted-decimal format, divided along 8-bit boundaries, and then converted to its decimal equivalent, separated by periods.

The following example shows a 128-bit IPv6 address in binary form:

0010000111011010000000001101001100000000000000000010111100111011
0000001010101010000000001111111111111110001010001001110001011010

The following example shows each 16-bit block in the address converted to hexadecimal and delimited with colons.

21DA:00D3:0000:2F3B:02AA:00FF:FE28:9C5A

Leading zeros can also be eliminated in IPv6. The following example shows the address without the leading zeros:

21DA:D3:0:2F3B:2AA:FF:FE28:9C5A

The technology that encapsulates IPv6 packets inside IPv4 packets is called tunneling. With IPv6, broadcasts are eliminated and replaced with multicasts.

OSI Model

There are seven layers in the Open Systems Inconnection (OSI) model:

  • The Application Layer provides network services to user applications. Some protocols at this level are FTP and SMTP.
  • The Presentation Layer is responsible for managing data and protocol conversion, compression, decompression, encryption, and decryption. It includes the following file formats: MPEG, MIDI, PICT, TIFF, JPEG, ASCII, and EBCDIC.
  • The Session Layer establishes, manages, and terminates sessions between nodes. It also handles security and name recognition. Protocols associated with this layer include NFS, SQL, RPC and AppleTalk.
  • The Transport Layer is responsible for setting up and defining how two nodes will communicate. The window works at this level to control how much information is transferred before an acknowledgement is required. This layer deals with segments and uses both connection-oriented (TCP) and connectionless protocols (UDP)
  • The Network Layer is responsible for path determination and for maintaining routing tables. Routers operate at this level.
  • The Data Link Layer is responsible for handling error notification, network topology and flow control. It is broken into two different layers: Media Access Control (MAC) and Logical Link Control (LLC). Bridges and switches operate at this layer.
  • The Media Access Control Sublayer builds frames from the 1’s and 0’s from the Physical layer.
  • The Logical Link Control Sublayer acts as a managing buffer between the upper layers and the lower layers. It is responsible for timing, and flow control.
  • The Physical Layer deals with physical transmission of data such as voltage, wiring and connectors.
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