1. Chapters 4 & 5 Addressing Will go over Exam 1
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Lecture

2. Recall, what info is used by router to make next hop decision ?
What ACTUAL info does the router use to make a next hop decision ?
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3. Mask
Given the network address, we can easily determine the block and range of addresses
Suppose given the IP address, can we determine the network address (beginning of the block) ?
To route packets to the correct network, a router must extract the network address from the destination IP address
For example, given , we know this is a class B, therefore is the netid and is the network address (starting address of the block)
How would we EXTRACT the network address from the IP address? We would use a MASK.
A mask is a 32-bit binary number that gives the first address in the block (the network address) when bitwise ANDed with an address in the block.
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4. AND operation
If bit is ANDed with 1, it’s preserved
If bit is ANDed with 0, it’s changed to a 0.
There are 3 default masks: one for each class. The default masks preserve the netid when ANDed with the addresses
Class A Default Mask:
Class B Default Mask:
Class C Default Mask:
A simple way to determine the netid for un-subnetted cases: (1) if mask byte is 255, retain corresponding byte of the address, (2) if mask byte is 0, set corresponding address byte to 0.
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Lecture

5. Examples
Given the address and the default class A mask, find the beginning address (network address).
The default mask is , which means that only the first byte is preserved and the other 3 bytes are set to 0s. The network address is
Solution
Given the address and the default class B mask, find the beginning address (network address).
The default mask is , which means that the first 2 bytes are preserved and the other 2 bytes are set to 0s. The network address is
Solution
Given the address and the class C default mask, find the beginning address (network address).
The default mask is , which means that the first 3 bytes are preserved and the last byte is set to 0. The network address is
Solution
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Lecture

6. 5-bit Address Space Illustration
No Netid case
32 addresses/block
Number of blocks: 1
Address range per block: 0 to 31
Netids: N/A
Network Addresses : 00000
Broadcast Addresses: 11111
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Lecture

7. 5-bit Address Space Illustration
1-bit Netid case
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
What is the mask ?
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Lecture

8. 5-bit Address Space Illustration
2-bit Netid Case
8 addresses/block
Number of blocks: 4
Address range per block: 0 to 7
Netids: 00, 01, 10, 11
Network Addresses : 00000, 01000, 10000, 11000
Broadcast Addresses: 00111, 01111, 10111, 11111
What is the mask ?
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Lecture

9. 5-bit Address Space Illustration
3-bit Netid Case
4 addresses/block
Number of blocks: 8
Address range per block: 0 to 3
Netids: 000, 001, 010, 011, 100, 101, 110, 111
Network Addresses : 00000, 00100, 01000, 01100
10000, 10100, 11000, 11100
Broadcast Addresses: 00011, 00111, 01011, 01111
10011, 10111, 11011, 11111
What is the mask ?
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Lecture

10. Mixing 3-bit & 2-bit Cases (think of the 32-bit case)
4 addresses/block and 8 addresses/block
Number of blocks: 6
Address range per block: 0 to 3 and 0 to 7
Netids: 000, 001, 010, 011, 10, 11
Network Addresses : 00000, 00100, 01000, 01100
10000, 11000
Broadcast Addresses: 00011, 00111, 01011, 01111
10111, 11111
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Lecture

11. Multihomed devices
As we mentioned that, 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
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12. 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
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.
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13. Example of this host on this address
IPless 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.
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.
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14. Example of loopback address
The IP address with the 1st 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.
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15. Sample internet With your new found knowledge, think about Project 2
Ethernet
ATM
Token Ring
Ethernet
With your new found knowledge, think about Project 2
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Lecture

16. Exam 2 Results & Grading Scale
Average Score = 48
Standard Deviation= 15 (large)
A-grade (0 students)
B-grade (7 students)
C-grade (4 students)
D-grade (5 students)
F-grade (1 student)
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