Saturday, March 24, 2012

Cisco CCNA: IP Addressing


Cisco CCNA: IP Addressing



Unit 1. The IP Address



In this course, you will learn about IP addressing. You will also be introduced to techniques that extend the IP address scheme such as subnetting and supernetting.
In this unit, you will learn about the basics of the IP address. You will see how IP addresses are split into different classes and how network devices recognize the class of an IP address.

After completing this unit, you should be able to:
  • Recognize an IP address

  • Correlate a binary IP address to a dotted-decimal address

  • Identify the different classes of IP addresses

  • List the features of each IP address class


This unit provides information that is relevant to the following CCNA exam objectives:
  • Describe the two parts of network addressing; then identify the parts in specific protocol address examples

  • Describe the different classes of IP addresses (and subnetting)


Topic 1.1: Overview of the IP Address

*Address Protocols
Network-layer addresses are used to route information across internets.
There are a few protocols used to control and maintain these addresses, such as IP, IPX, and AppleTalk.
IP (Internet Protocol), which is part of the TCP/IP suite of protocols, is by far the most widely used of these protocols.

*Common Address Method
The widespread popularity of IP provides a common method for distributing and using network addresses for today's Internet. IP allows users to access data from faraway hosts that may be operating on different network types and media.

*The IP Address
At the core of IP is the IP address. An IP address consists of 32 binary bits, or four bytes. Each byte is considered an octet, since it contains eight bits. Each octet represents a separate number from 0 to 255.

Topic 1.1.1: Dotted-Decimal Notation

*Binary to Decimal
It is hard for the human eye to decipher a binary IP address quickly.
Therefore, a method called dotted-decimal notation is used to transform the binary IP address into a decimal IP address.
Methods to convert numbers from binary to decimal and from decimal to binary are shown in another course in this series.

*Dotted-Decimal Format
In dotted-decimal notation, each octet is converted into its decimal equivalent and separated by periods. Therefore, a binary IP address of 11000001001000100010101000000010 would yield a dotted-decimal value of 193.34.42.2.

*Machines Use Binary
An address of 193.34.42.2 is much easier for a human to read and understand than 11000001001000100010101000000010. There are also text-based addresses, such as yahoo.com that are even easier for humans to comprehend. However, dotted-decimal and text-based addresses are always converted to binary before they can be used by network devices.

Topic 1.1.2: Networks and Hosts

*Destination
IP addresses contain information pertaining to two different destinations. One of these destinations is the particular network for which an IP packet is intended, and the other is the particular host or node for which an IP packet is intended.

*Address Partition
The amount of the address that pertains to the network and that which pertains to the host is variable. One way to denote how much of the address pertains to each portion is to append the address with a slash followed by the number of bits reserved for the network. For example, 183.71.200.243/16 would indicate that 16 bits of the address belongs to the network portion. Another method is to divide IP addresses into different classes.

Question 1

Question 2

Question 3

Topic 1.1.3: Classes of IP Addresses

*Classful Addressing
Separating the IP addresses into different classes provides organizations with the flexibility to support networks of varying sizes. This use of address classes is called classful addressing. There is also a classless addressing scheme, which is presented in another unit of this course.

*IP Address Classes
There are currently five classes of IP addresses: Classes A, B, C, D, and E.
Each of these classes is identified by the amount of the IP address reserved for the network and the amount reserved for the host.

*Address Calculation
The amount of space in the IP address reserved for the network and host determines the number of networks and hosts that each class can support.
This number is found by using the number of variable reserved bits as the exponent of 2.
An IP address with eight bits reserved for the host address has 2^8 (or 256) possible hosts (from 0 to 255).

*Subtract 2
Even though there are 256 possible hosts in the previous example, two of these hosts may not be used.
An octet of all 0s (0), and an octet of all 1s (255), are reserved for the default route (or wire address) and the broadcast address of the network, respectively.
So there are only 254 hosts available with eight variable bits.

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*Number of Class A Addresses
The first bit of a Class A address is always 0. This leaves 31 variable bits, giving 2^31 (2,147,483,648) possible addresses, which is 50% of the total number of IP addresses that are possible with 32 bits (2^32, or 4,294,967,296).

*Class A Format
The Class A address is of the form network.host.host.host. Thus, a Class A address has eight network bits (only seven of which are variable) and 24 host bits, providing 126 networks with 16,777,214 hosts per network.

*Who Gets Class A
Since there are only 126 network numbers in a Class A address, these addresses are reserved for very large organizations.
It would make very little sense to give a company of 10 users an IP network address that can support more than 16 million hosts.

*Number of Class B Addresses
The first two bits of a Class B address are always 10. This leaves 30 variable bits, giving 2^30 (1,073,741,824) possible addresses, which is 25% of the total number of IP addresses.

*Class B Format
The Class B address is of the form network.network.host.host. Thus, a Class B address has 16 network bits (only 14 of which are variable) and 16 host bits, providing 16,382 networks with 65,532 hosts per network.

*Who Gets Class B
Class B addresses are used for medium-sized organizations.

*Number of Class C Addresses
The first three bits of a Class C address are always 110. This leaves 29 variable bits, giving 2^29 (536,870,912) possible addresses, which is 12.5% of the total number of IP addresses.

*Class C Format
The Class C address is of the form network.network.network.host. Thus, a Class C address has 24 network bits (only 21 of which are variable) and 8 host bits, providing 2,097,152 networks and 254 hosts per network.

*Who Gets Class C
Class C addresses are used in small organizations with less than 254 hosts. However, multiple Class C addresses can be assigned to the same organization to allow for more than 254 hosts.

*Multicasting and Experimenting
Class D and E addresses are not assigned to any organizations. Class D addresses begin with 1110, and are used for multicast purposes. Class E addresses begin with 1111, and are reserved for experimental purposes. Together, these addresses account for the remaining 12.5% of IP numbers.

Question 7

Topic 1.1.4: First Octet Rule

*Determining Address Class
The First Octet Rule allows a network device to determine the class of an IP address by reading the first octet of the address. According to this rule, the first octet of addresses from each class must fall within a specified range.

*Octet Value Ranges
The range of first octet values allowed for each class (in decimal format) are as follows:
  • Class A: 1-126

  • Class B: 128-191

  • Class C: 192-223

  • Class D: 224-239

  • Class E: 240-255


*Read and Route
Once a network device reads the first octet of an IP address, it determines how many more bits (if any) it must read before it can make a routing decision.

*Loopback Address
The first octet value of 127 is not assigned to any particular class.
This is because the network 127.0.0.0 is used as the loopback address.
The loopback address is used for internal addressing in a device to check for stack corruption in the TCP/IP stack.

Question 8

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Question 10


* Exercise 1
Try making a table that contains the different IP address classes along with their first octet in decimal and binary.


Examine the following table
Step Action
1 Draw a table with 3 columns and 5 rows.
2 Label the columns "IP Address Class," "First Octet Decimal," and "First Octet Binary."
3 Complete the first column with the different IP address classes.
4 For each IP address class in the first column, enter the range of decimal values allowed as the first octet for that class in the second column.
5 Convert the values in the second column to binary, and place the binary value in the third column.
6 Try to memorize the table, and keep the table for future reference.


Topic 1.2: Unit 1 Summary

In this unit you learned the basics of the IP address. You saw how the binary IP address can be represented with dotted-decimal notation. You learned about the various classes of IP addresses and how to recognize them. Finally, you were introduced to the First Octet Rule that network devices use when they read IP addresses.
In the next unit of this course, you will be introduced to the concept of subnetting.

Unit 2. Subnetting



In this unit, you will learn about subnetting and how subnetting is used to overcome some of the limitations of classful addressing. You will learn how masking works and you will see the difference between network masks and subnet masks.
You will also be introduced to the process for subnet planning, and you will see specific examples to illustrate the concepts of the planning process.

After completing this unit, you should be able to:
  • List the limitations of classful addressing

  • Recognize a subnet mask

  • Identify all components of a subnet address

  • Detail a subnet plan


This unit provides information that is relevant to the following CCNA exam objectives:
  • Describe the two parts of network addressing; then identify the parts in specific protocol address examples

  • Describe the different classes of IP addresses (and subnetting)


Topic 2.1: Problems with IP Addresses

*Internet Is Huge
The Internet has been rapidly expanding for a number of years.
Due to this expansion, the number of IP addresses required by Internet users has risen dramatically.
ICANN (Internet Corporation for Assigned Names and Numbers), the organization that manages and assigns IP addresses, is facing many problems associated with the increased demand for these addresses.

*Limitation of 32 Bits
One problem is that the 32-bit IP addressing scheme only provides for 2^32 (4,294,967,296) addresses. This is a large number of addresses, but there is also a large demand for them. As this demand rapidly increases, the number of available addresses dwindles.

*Address Allocation
Another problem is that addresses have been distributed based on request, rather than need.
For example, an organization of 150 users might have a class B address (providing for up to 65,534 hosts), when one class C address (providing for up to 254 hosts) would provide a more efficient allocation of address space.

*Problems with Classful Addressing
There is also the problem of the classful addressing scheme itself. While this scheme is easy to understand and implement, it is not very efficient. For instance, an organization of 3,500 users could be assigned one class B address, which would be very inefficient, or it could be assigned 14 class C addresses, which would create 14 different Internet routing table entries to the same organization.

Topic 2.2: Subnet Design

*Growing Tables
As the Internet grows, more addresses are needed to service the growing Internet population.
Routers use these addresses to make routing decisions, and the more addresses are in use, the larger routing tables become.
As these tables grow, routers take longer and longer to make routing decisions.

*Overcoming Classful Addressing
Subnetting is a process that was designed to overcome some of the problems associated with the classful addressing system. The most significant of these problems concern the growing size of the Internet routing tables and the need for additional network numbers for medium-sized organizations.

*Another Hierarchy
Subnetting provides an additional level of hierarchy to the classful addressing system.
By creating subnets, an administrator can set up as many internal networks as needed, while advertising only one network to external routers.
This allows a complex internal network while keeping Internet routing tables to a minimum number.

*Limited Address Space
Another benefit of subnetting is that it allows administrators to use their limited address space more efficiently. An administrator with one class C IP address can subnet that address instead of requesting another IP address from ICANN.

*Splits Host
Subnets are created by dividing the host portion of a classful address into a subnet portion and a host portion. The number of bits set aside for the subnets and hosts is variable, and requires some prior planning.

*Masking Number
With subnets, routers need to know which part of the IP address is for the network, subnet, and host. This led to the development of masking, which uses a 32-bit number with a one-to-one correspondence to the IP address. This number allows routers to determine which part of the IP address to use for the different hierarchies.

Question 11

Question 12

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Question 14

Topic 2.2.1: Masking

*AND Operation
A mask uses 1s and 0s, just like an IP address. However, in a mask, a 1 indicates that the corresponding bit in the IP address should be read, while a 0 indicates that the corresponding bit in the IP address should be ignored. This is a simple AND operation. For example, a router reading the IP address 155.32.215.4 with a subnet mask of 255.255.192.0 results in an address of 155.32.192.0, as shown below.

*Hiding Subnet Structure
Masking also allows administrators to hide their internal subnet structure from outside routers. All external data is delivered to one IP address (or at most a few IP addresses). This also adds an extra level of security for the internal network.

*Routing Table Entries
Since masks hide internal networks from outside routers, the routing tables of Internet routers will not be affected by the size or complexity of an organization's internal network. Internet routers will only have one entry (or at most a few entries) for any particular organization.

*Differentiating between Subnets
Internal routers have table entries for all the subnets in a network. Each subnet has a mask associated with it. The router then applies the mask to the address in order to identify the subnet.

Topic 2.2.2: Default Network Mask

*Network Mask
The network mask is the default mask of an IP address. It corresponds with the classful address system and there is a different mask for each class. Subnet masks, discussed later, add to these default network masks.

*Class A Network Mask
Class A IP addresses have a network mask of 255.0.0.0. Only the first octet of a class A IP address is reserved for the network. Therefore, the mask 11111111 00000000 00000000 00000000 tells the router to read only the first octet.

*Class B Network Mask
Class B IP addresses have a network mask of 255.255.0.0. This mask lets the router know that it only needs to read the first two octets of the address to identify the network.

*Class C Network Mask
Class C IP addresses have a network mask of 255.255.255.0. This lets the router know that it needs to read the first three octets of the address to identify the network.

Question 15

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Question 18

Topic 2.2.3: Subnet Mask

*Subnet Bits
The subnet mask is used to identify the bits used for the subnets of a network. These bits are borrowed from the host portion of the IP address. These bits are always the furthest on the left of the host portion of the address.

*Reading and Ignoring
In addition to the classful network mask, the subnet mask also identifies the host bits that are used for subnetting. The subnet portion of the mask also uses 1s to indicate that the corresponding bit on the IP address should be read by the router. When the router is looking for a subnet, it will read these bits when making the routing decision. The remaining 0s let the router know to ignore the bits for individual hosts.

*Decimal Equivalents
When dealing with subnets, it is important to know the decimal value of contiguous 1s in an octet.
Each bit of an octet has a corresponding decimal value.
To determine the decimal value of an entire octet, the value of the individual bits must be added together.

*Network, Subnet, Host
To get the entire subnet mask, append the subnet and host portions of the mask to the network portion of the mask. For example, a class B IP address has a default network mask of 255.255.0.0. If five bits of the host portion of the IP address are used for subnetting, the third octet of the mask is 11111000 (248). Therefore, the entire subnet mask is 255.255.248.0.

*Determining What to Read
To find a subnet and a host using an IP address and a subnet mask, a router performs the AND operation. For instance, the IP address 201.32.156.215 (a class C address) with the subnet mask 255.255.255.224 sends a packet to network 201.32.156.0, subnet 192. The remaining bits (10111) identify host 23.

*Extended Network Prefix
The address 201.32.156.215, subnet 255.255.255.224 can also be expressed in the /n notation (extended network prefix), where n is the number of bits in the network and subnet portions of the address. Since a mask of 255.255.255.224 borrows three bits from the host portion and there are 24 bits in the network portion of the address, the IP address can be expressed as 201.32.156.215/27. This notation is only for human readability, however. Routers still use binary subnet masks.

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Question 20

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Topic 2.2.4: Subnet Address

*Number of Subnets
All 1s or all 0s are generally not used for subnet portions of the address because of possible conflicts with protocols using the classful addressing system.
Therefore the number of possible subnets may be calculated as (2^x – 2), where x is the number of borrowed bits used for subnetting.

*Number of Hosts
The number of host addresses available on each subnet may be calculated as (2^y – 2), where y is (number of host bits) – (number of subnet bits).

*Wire and Broadcast Addresses
Each subnet also has a wire (network) address and a broadcast address. These addresses are identified by all 0s (wire address) or all 1s (broadcast address) in the host portion of the address.

*Class C Subnet Example
The IP address 201.32.156.215/27 has a first octet value of 201, making it a class C address. Class C addresses have 24 bits reserved for the network, and the /27 indicates that an additional three bits are used for subnetting. Therefore, the subnet mask has 27 contiguous 1s followed by five 0s, which yields a subnet mask of 255.255.255.224. The wire address for this subnet is then 201.32.156.192 (host portion all 0s), the broadcast address is 201.32.156.223 (host portion all 1s), and valid host addresses are 201.32.156.193 through 201.32.156.222.

*Class B Subnet Example
In a class B IP address, there are two octets to split between subnets and hosts. The IP address 132.128.214.78/21 has a subnet mask of 255.255.248.0, since it has five borrowed bits for the subnet. The subnet wire address is then 132.128.208.0, the broadcast address is 132.128.215.255, and valid host addresses are 132.128.208.1 through 132.128.215.254.

Question 23

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Question 26

Topic 2.3: Subnet Planning

*Considerations
When devising an addressing plan, it is important to know how many subnets are needed and the largest number of hosts to be placed on any given subnet. Future growth should also be taken into consideration in order to ensure a long life span for the addressing plan.

*Steps of Subnet Planning
When planning for subnets, the following steps should be taken:
  • Define the mask

  • Define subnet numbers

  • Define hosts for each subnet

  • Define the broadcast address for each subnet


Topic 2.3.1: Class C Example

*Define the Mask
The mask is defined by determining how many bits need to be borrowed from the host portion of the address and used for subnetting. Enough bits must be borrowed to ensure all needs are met. For example, if an organization with a class C address needs five subnets, at least three bits must be used for subnetting [(2^3 - 2) = 6]. This would leave one unused subnet for future growth. The subnet mask would then be 255.255.255.224.

*Define Subnet Numbers
Each subnet created must be defined by a unique address called the wire address, which is a combination of the network portion of the IP address and the subnet number. The possible subnet numbers using three borrowed bits is 001 (1), 010 (2), 011 (3), 100 (4), 101 (5), and 110 (6). However, these numbers are on the left of an octet, and correspond to decimal values of 32, 64, 96, 128, 160, and 192, respectively. Therefore, the third subnet (011, or 96) of the network 201.16.74.0 would have an address of 201.16.74.96.

*Define Hosts for Each Subnet
Individual hosts are also uniquely defined. Administrators must assign host numbers, and append them to the network and subnet portion of an IP address. Administrators usually follow a sequential order when assigning hosts to a subnet, but this is not required. Valid host numbers may be any number that fits within the host portion of the address except for the all 0s and all 1s addresses. Valid host addresses for subnet 201.16.74.96 would be 201.16.74.97 through 201.16.74.126.

*Define Broadcast Address for Each Subnet
Each subnet has a separate broadcast address. This is the address with all 1s in the host portion of the address. The broadcast address for subnet 201.16.74.64 would be 201.16.74.127.

Question 27

Question 28

Question 29

Topic 2.3.2: Class B Example

*Class B Planning
Class B IP addresses are a little more complicated.
For example, if network 172.28.0.0 needed 300 subnets, with 100 hosts per subnet, nine bits must be borrowed from the host portion of the address.
This would allow 510 subnets (2^9 – 2) and 126 hosts per subnet (2^7 – 2).
Borrowing eight bits would only allow 254 subnets and borrowing 10 bits would only allow 62 hosts per subnet.

*Class B Subnet Mask
The subnet mask would then be 25 contiguous 1s (16 bits for the class B network mask and 9 bits for the subnet) followed by 7 contiguous 0s. This yields a subnet mask of 255.255.255.128.

*Class B Subnet Number
The subnet numbers for this example stretch over two octets (all of the third octet and the first bit of the fourth octet). Since the all 0s subnet cannot be used, the first usable subnet is 172.28.0.128. Since the all 1s subnet cannot be used, the last usable subnet is 172.28.255.0.

*Class B Hosts
Since there are seven bits left for the host, there are (2^7 – 2), or 126 possible hosts, with valid host numbers ranging from 1-126 on each subnet. For example, host 36 of subnet 9 would have the address 172.28.4.164.

*Class B Broadcast Addresses
The broadcast address for each subnet would have all 1s in the host portion of the address. Therefore, the broadcast address for the first subnet is 172.28.0.255, the second is 172.28.1.127, the third 172.28.1.255, etc.


* Exercise 1
Try becoming familiar with binary octets and their decimal equivalents.

Examine the following table
Step Action
1 Draw a table with nine columns and 20 rows.
2 Label the column headings 128, 64, 32, 16, 8, 4, 2, 1, and Decimal Equivalent.
3 Randomly place 1s and 0s in all the empty cells of the first eight columns, leaving column 9 empty.
4 You now have a list of 20 octets. Convert each octet to its decimal equivalent by adding the value of the column heading for each instance of 1 in the row and place the total in the ninth column.
5 Find a list of binary and decimal equivalents (in a book or on the Internet) and compare your answers with the listed values.
6 If any of your answers were incorrect, try this exercise again.


Topic 2.4: Unit 2 Summary

In this unit you learned about subnetting. You saw how subnetting was designed to overcome classful addressing limitations. You were introduced to network masks and subnet masks. You also learned to recognize the different components of a subnet, as well as how to plan subnets to allow for current as well as future needs.
In the next unit of this course, you will be introduced to supernetting.

Unit 3. Supernetting



In this unit, you will learn about the concept of supernetting. You will see how supernetting is designed to overcome the limitations of the classful IP addressing system.
You will also be introduced to Classless InterDomain Routing (CIDR). You will see how CIDR reduces routing table entries while allowing for exceptions. You will also learn some considerations for CIDR implementation.

After completing this unit, you should be able to:
  • List the benefits of supernetting

  • Recognize network prefixes

  • Divide address blocks

  • List the requirements for CIDR


This unit does not address any specific Cisco objectives. However, it does provide background information that is essential for the CCNA exam.
In the course index, questions about background information are indicated with the abbreviation BCK and a short description of the question subject matter.

Topic 3.1: Problems of Internet Growth

*Problems with Classful Addressing
While subnetting helps alleviate some of the problems of the classful addressing system, some problems still exist. These problems include the exhaustion of class B addresses and the huge increase in the number of Internet routing table entries.

Topic 3.1.1: Class B Exhaustion

*Unused Class B Addresses
Class B addresses provide for up to 65,534 individual hosts for each address.
In the past, class B addresses were assigned to fairly small organizations, which left a large number of possible host addresses unused.

Topic 3.1.2: Routing Table Explosion

*Multiple Table Entries
As class B addresses became scarce, medium-sized organizations were assigned several class C addresses. This created several Internet routing table entries for the same organization.

*Increasing Table Size
The Internet has also gone through a period of exponential growth, and the size of Internet routing tables has increased dramatically.

Topic 3.2: Introduction to CIDR

*Supernetting
To address the problems of class B address exhaustion and large Internet routing tables, the Internet community has adopted the process of supernetting.
This process is also called CIDR (Classless Inter-Domain Routing).

*Advertising Groups
CIDR allows administrators to advertise a group of IP addresses as one address. In this process, called route aggregation, one routing table entry could represent many addresses.

*The Network Prefix
With CIDR, the classful addressing system is superceded by the use of a network prefix. Instead of using the First Octet Rule to see if the network portion of the IP address is 8, 16, or 24 bits long, routers using CIDR read the network prefix to determine how many bits of an IP designate the network.

*Variable Lengths
The use of a network prefix allows the network portion of an IP address to be any size. The network prefix is used to show how many bits are set to 1 in the network mask. The IP address 204.78.20.243/14 has a network prefix of 14, which means that the network mask has 14 bits set to 1.

*Comparing CIDR with Classful Addressing
In the classful addressing system, the IP address 204.78.20.243 would be one class C address with 24 bits to indicate the network and eight bits reserved for the host. However, an organization with the address 204.78.20.243/14 would have 18 host bits, which gives the equivalent address space of 1024 class C addresses.

*Flexible Network Design
CIDR is very efficient in allocating addresses. The use of the network prefix provides administrators with greater flexibility in designing networks.

Question 30

Question 31

Topic 3.3: Uses of CIDR

*Individual Assignments
The address 201.32.0.0/16 provides the equivalent of 256 class C addresses.
In a classful system, these addresses would have to be individually allocated to organizations.

Topic 3.3.1: Allocating Addresses

*ISP Example
An ISP (Internet Service Provider) with the address block 201.32.0.0/16 can divide this block into many smaller blocks designed to meet the network needs of its clients. For instance, take three ISP clients. Company A has 25,000 users, Company B has 14,000 users, and Company C has 3,000 users.

*Assigning Address Blocks
The ISP can assign the address block 201.32.0.0/17 to Company A, 201.32.128.0/18 to Company B, and 201.32.224.0/20 to Company C. This would satisfy each company's current needs and provide room to grow. It would also leave the ISP with plenty of addresses to use for its own growth.

Topic 3.3.2: Routing Domains

*Dividing the Internet
Another use of CIDR is to control the growth of Internet routing tables by dividing the Internet into routing domains. All network devices within a domain have routing table entries for each other. Network devices outside the domain, however, are only aware of the network prefix for the entire domain.

*Reducing Table Entries
The use of domains reduces the number of Internet routing table entries. In the previous example, Company A had the equivalent of 128 class C addresses, Company B had the equivalent of 64 class C addresses, and Company C had the equivalent of 16 class C addresses, all of which were administered by an ISP having the equivalent of 256 class C addresses.

*Advertising One Address
The routers of each individual company in the example keep track of all the IP addresses within the company itself, while advertising only one IP address to the other companies in the domain. The routers of the ISP have one entry for each of the companies, but advertise only one address to the Internet. Thus, Internet routers have one table entry to represent all the networks in the ISP's domain.

Question 32

Question 33

Question 34

Topic 3.3.3: Exceptions

*Switching ISPs
Suppose, however, that Company B wants to switch to a different ISP. To keep routing tables small, they should change their IP addresses to the ones offered by the new ISP.

*Using Exceptions
This rarely occurs, however, because changing the IP addresses of all hosts in an organization is a major task. What is done instead is to make an exception, where the second ISP advertises Company B's address space (201.32.128.0/18) along with its own address space. This means that there are now two routes to the 201.32.0.0 address space instead of the one route previously used.

*Negating CIDR Benefits
Exceptions should be avoided where possible, because it reduces the effectiveness of CIDR. One of the major benefits of CIDR is the reduction of routing table entries, and too many exceptions would negate this benefit.

Topic 3.4: CIDR Considerations

*Problems with Protocols
CIDR may cause problems with hosts using classful routing protocols.
These problems occur because classful routing protocols do not allow network portions of addresses to be shorter than their classful equivalent.

*Choosing a Protocol
To ensure that networks support a classless addressing system, routing protocols should be chosen that supports CIDR. RIPv2, OSPF, and BGP-4 are three routing protocols that support CIDR.

*Extending IPv4 Space
CIDR extends the life of the IPv4 address space. CIDR allows the Internet community to use its limited address space more efficiently and extend the life span of IPv4. In the future, IP version 6 (IPv6) will have 128-bit addresses to provide for projected growth of the Internet.

Question 35

Question 36

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Question 38


* Exercise 1
Try designing an Internet domain.

Examine the following table
Step Action
1 Draw one large oval containing five smaller ovals.
2 Label the large oval ISP-1 and the smaller ovals Company A, Company B, Company C, Company D, and Company E.
3 Decide how many IP addresses you want each company to have and place that number in the corresponding oval.
4 Assign the address block 204.96.0.0/14 to ISP-1.
5 On the basis of the number you placed in each company, assign address blocks from the ISP to each company that will not waste addresses but which are sufficient for the company's needs.
6 Repeat steps 1-5 for ISP-2 with the address block 198.224.0.0/14 and containing companies F, G, H, I, and J.
7 Move Company C to ISP-2 and Company I to ISP-1. Keep the original IP address block for each company.
8 Create routing tables for each company and ISP.


Topic 3.5: Unit 3 Summary

In this unit you learned about supernetting and CIDR. You saw how CIDR reduces routing table entries and provides a more efficient use of the limited IPv4 address space.
In this course, you learned about classful IP addresses. You saw how the classful addressing system had severe limitations, and how these limitations can be overcome by subnetting and supernetting.

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