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Dr. Clincy Lecture 1

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Multihomed devices
• As we mentioned, any device with one or more connections to the
Internet will need an IP address for EACH connection – such devices are called “multihomed” devices.
• A Router could be a multihomed device
Dr. Clincy Lecture 2

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Example of direct broadcast address
Router sending to all hosts on a network
If the hostid is all 1’s, it’s called a
“broadcast address” and the router use it to send a packet to all host in a specific network. In this case, hosts 20, 64, 126 and etc. will receive the packet from the router
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Example of limited broadcast address
Host sending to all other hosts on a network
If the hostid and netid are all 1’s, it’s called a “limited broadcast address” . If the host wants to send a packet to all host in a specific network, it would use this address.
The router would block this address so that data stays contained within a specific network.
Dr. Clincy Lecture 3

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Example of this host on this address
IP-less Host sending message to bootstrap server
An address of all 0’s is used during bootstrap time if the host doesn’t know it’s IP address. The un-named host sends an all 0 source address and limited broadcast (all 1’s) destination address to the bootstrap server.
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Example of specific host on this network
Host sending to some other specific host on a network
An address with a netid of all 0’s is used by a host or router to send another host with in the same network a message.
Dr. Clincy Lecture 4

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Example of loopback address
• The IP address with the 1 st byte equal to 127 is used for the loop back address.
• Loopback address is used to test software on a machine – the packet never leaves the machine – it returns to the protocol software
• Example: a “ping” command can send a packet with a loopback address as the destination address to see if the IP software is capable of receiving and processing a packet.
Dr. Clincy Lecture 5

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Token Ring
Sample internet
ATM
Ethernet
Ethernet
Dr. Clincy Lecture 6

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Dr. Clincy Lecture 7

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• When we talked about CLASSFUL addressing – we realized the problem of wasted host addresses and depleting available network addresses .
• In subnetting, a network is divided into several smaller networks called subnetworks or subnets – each subnet will have it’s own address
• Typically, there are 2 steps in reaching a destination: first we must reach the network (netid) and then we reach the destination (hostid)
Dr. Clincy Lecture 8

A network with two levels of hierarchy (not subnetted)
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0 The 2 level approach is not enough some times – you can only have 1 physical network – in example, all host are at the same level – no grouping
Dr. Clincy Lecture 9

A network with three levels of hierarchy (subnetted)
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With subnetting, hosts can be grouped
(0-63)
(192-255)
(64-127)
(128-191)
Dr. Clincy Lecture 10

Addresses in a network with and without subnetting
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With subnetting, there are 3 levels (versus 2 levels).
Partition the hostid space into subnetid and hostid.
(1 st ) network, (2 nd ) subnetwork and (3 rd ) host
Dr. Clincy Lecture 11

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Similar to Hierarchy concept in a telephone number
Dr. Clincy Lecture 12

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Default mask and subnet mask
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Finding the Subnet Address
Given an IP address, we can find the subnet address the same way we found the network address in the previous chapter. We apply the mask to the address. We can do this in two ways: straight or short-cut.
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Straight Method
In the straight method, we use binary notation for both the address and the mask and then apply the AND operation to find the subnet address.
Short-Cut Method
** If the byte in the mask is 255, copy the byte in the address.
** If the byte in the mask is 0, replace the byte in the address with 0.
** If the byte in the mask is neither 255 nor 0, we write the mask and the address in binary and apply the AND operation.
Dr. Clincy Lecture 14

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• In the early days, non-contiguous 1’s masks were used (0’s and 1’s could alternate)
• Today, as a best practice, contiguous 1’s masks are used
• In either case, the black box can perform the “masking” process
Dr. Clincy Lecture 15

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Example 1
What is the subnetwork address if the destination address is 200.45.34.56 and the subnet mask is 255.255.240.0?
Solution
11001000 00101101 00100010 00111000
11111111 11111111 1111 0000 00000000
11001000 00101101 0010 0000 00000000
The subnetwork address is 200.45.32.0
.
Dr. Clincy Lecture 16

Recall - 5-bit Address Space Illustration
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16 addresses/block
Number of blocks: 2
Address range per block: 0 to 15
Netids: 0, 1
Network Addresses : 00000, 10000
Broadcast Addresses: 01111, 11111
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Dr. Clincy Lecture 17

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5-bit Address Space Illustration subnet
1-bit Subnet case
Number of blocks/networks: 2
Number subnets per block: 2
8 addresses/subnet
Address range per subnet: 0 to 7
Subnet ids: 0, 1
Network Addresses : 00000, 01000, 10000, 11000
Broadcast Addresses: 00111, 01111, 10111, 11111
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Dr. Clincy Lecture 18

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5-bit Address Space Illustration subnet
2-bit Subnet case
Number of blocks/networks: 2
Number subnets per block: 4
4 addresses/subnet
Address range per subnet: 0 to 3
Subnet ids: 00, 01, 10, 11
Network Addresses : 00000, 00100, 01000, 01100
10000, 10100, 11000, 11100
Broadcast Addresses: 00011, 00111, 01011, 01111
10011, 10111, 11011, 11111
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Dr. Clincy Lecture 19

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Illustrating the mask concept (1 of 3) netid
What is the mask ? 10000
If address 11101 is masked, what is the result ?
result address
Lecture
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Illustrating the mask concept (2 of 3) subnet
What is the mask (subnet mask) ? 11000
If address 11101 is masked, what is the result ?
result address
Lecture
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Illustrating the mask concept (3 of 3) subnet
What is the mask (subnet mask) ? 11100
If address 11101 is masked, what is the result ?
result address
Lecture
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Example 2
What is the subnetwork address if the destination address is 19.30.84.5
and the mask is
255.255.192.0?
Dr. Clincy Lecture 23

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Comparison of a default mask and a subnet mask
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A portion of the hostid space is divided between some contiguous 1’s and 0’s
Dr. Clincy Lecture 24

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The number of subnets must be a power of 2.
Determine the number of subnets added by looking at the number of 1s added to the default mask and performing 2 raised to that number
For example, 2 3 = 8 subnets
Dr. Clincy Lecture 25

Example 3
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A company is granted the site address
201.70.64.0 (class C). The company needs six subnets. Design the subnets.
Solution
The number mask is 24 (class C).
of 1s in the default
The company needs six subnets. This number 6 is not a power of 2. The next number that is a power of 2 is 8 (2 3 ). We need 3 more 1s in the subnet mask. The total number of 1s in the subnet mask is 27 (24 + 3).
The total number of 0s is 5 (32 27). The mask would be
Dr. Clincy Lecture 26

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Solution (Continued)
11111111 11111111 11111111 111 00000 or
255.255.255.224
The number of subnets is 8.
The number of addresses in each subnet is 2 5 (5 is the number of 0s) or 32.
Dr. Clincy Lecture 27

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Dr. Clincy
Example 3
Lecture 28

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Example 4
A company is granted the site address 181.56.0.0
(class B). The company needs 1000 subnets. Design the subnets.
Solution
The number of 1s in the default mask is 16 (class B).
The company needs 1000 subnets. This number is not a power of 2. The next number that is a power of 2 is 1024 (2 10 ). We need 10 more 1s in the subnet mask.
The total number of 1s in the subnet mask is 26 (16 + 10).
The total number of 0s is 6 (32 26).
The mask is
11111111 11111111 11111111 11000000 or
255.255.255.192.
The number of subnets is 1024.
The number of addresses in each subnet is 2 6 (6 is the number of 0s) or 64.
Dr. Clincy Lecture 29

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Dr. Clincy
Example 4
Lecture
Subtract 63 from 255 to get 192
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