1. 4.3 Network Layer Logical Addressing
communication at the network layer, is a host-to-host (computer-to-computer) communication – a host communicate with another host somewhere in the world
for that, a global addressing scheme is necessary
logical addressing at network layer (TCP/IP suite) = IP address
there are current two version of IP addressing
IPv4 : IP version 4
IPv6 : IP version 6
IPv4 is a 32 bits long address that uniquely and universally defines the address of the host in the internet
no two device on the internet can have the same IP address at the same time
a device operating at the network layer has m connections to the internet, needs to have m address
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2. 4.3 Network Layer Logical Addressing - Intro
Address space
an address space is the total number of addresses used by the protocol
Ex : IPv4 is a 32 bits long address, so the total address space is 232 or 4,294,967,296 (more than 4 billions)
Address notation
Binary notation –
Dotted-Decimal Notation –
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3. 4.3.1 Classful Addressing IPv4 addressing use the concept of classes
currently becoming obselete
in classful addressing, the address space is divided into 5 classes : A, B, C, D, E
each class occupies some part of the address space
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4. 4.3.1 Classful Addressing Classes and Blocks
each class is divided into a fixed number of blocks with each block having a fixed
Class A address – assigned for large organizations with large number of host/route
Class B address – midsize organization with tens of thousands host
Class C address – small organizations with small number of hosts
Class D address – multicasting
Class E address – future use
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5. 4.3.1 Classful Addressing What wrong with classes and blocks ?
large part of the available addresses were wasted
class A address is two large for any organization – most were wasted and not used
class B – still too large for most organizations
class C – too small for many organization
class D – Internet authorities wrongly predicted (never happened)
class E – only a few were used.
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6. 4.3.1 Classful Addressing Netid and HostId
in classful addressing, IP address in Class A, B and C is devided into netid and hostid.
varying length, depending on the class of the address.
netid shows which network the host belongs to
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7. 4.3.1 Classful Addressing Mask
is a 32 bits number made of contiguous 1s (together in sequence), followed by contiguous 0s.
does not apply for class D and E.
a mask can be used to find the netid and hostid
Ex : the mask for a class A address has eight 1s, means the first byte of any address in class A define the netid, the next 24 bits define the hostid.
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8. 4.3.1 Classful Addressing Slash Notation Address Depletion
Also known as CIDR (Classless Inter Domain Routing) notation
A.B.C.D/n : n is the number of contiguous 1s in the mask
Ex : /24 :mask for this address is
For classful addressing the normal value is 8, 16 and 24
Address Depletion
the flaws in the classful addressing scheme + fast growth of internet led to the near depletion of the available address
class A and B addresses are almost run out, and class C addresses are too small for most midsize organizations.
as a solution – classless addressing
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9. 4.3.2 Classless Addressing
created to overcome the address depletion in classful addressing
more address means more computers/network/organizations can join the internet
addresses are granted in blocks
Address blocks
an organization (small or large) is granted a block (range) of address
size of the block (number of address) is based on the size of the entity
Restriction in block addressing
address in a block must be contiguous, one after another
the number of address in a block must be a power of 2 (1, 2, 4, 8, ....)
the first address must be evenly divisible by the number of address
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10. 4.3.2 Classless Addressing Restriction in block addressing (Ex :) Mask
a mask is a 32 bits number in which the n leftmost bits are 1s and the 32-n rightmost bits are 0s.
in classless the mask for a block of address can take any value from 0 to 32
normally written in slash (CIDR) notation
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11. 4.3.2 Classless Addressing Mask
Searching for the first address in a block of addresses
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12. 4.3.2 Classless Addressing
Searching for the first address in a block of addresses
Ex : A block of addresses is granted to a small organization. We know that one of the addresses is /28. What is the first address in the block?
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13. 4.3.2 Classless Addressing
Searching for last address in a block of addresses
Ex : A block of addresses is granted to a small organization. We know that one of the addresses is /28. What is the last address in the block?
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14. 4.3.2 Classless Addressing Number of addresses in a block
the number of addresses in the block is the difference between the last and the first address
Ex : (from the previous example)
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15. 4.3.2 Classless Addressing
Another method of finding the first, last & number of address in a block
Ex : given one of the address in the block is /28 (same as previous example)
1. represent the mask as a 32 bits binary number
2. first address can be found by ANDing the mask with the address
3. the last address can be found by ORing the mask complement with the address
4. the number of address can be found by complementing the mask, change it as a decimal number and add 1 to it.
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16. 4.3.3 Network Addresses
when an organization is given a block of addresses, the organization is free to allocate the addresses to the devices that need to be connected to the internet
the first address in the block is normally (not always) not assigned to any device in the network
the first address is called as the network address and defines the organization network
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17. 4.3.3 Network Addresses
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18. 4.3.4 IP Sub-networking (Subnetting)
what actually first address means in the network ?
first address = network address
last address = broadcast address
number of valid address in a block = number of available – network address – broadcast address
network can be divided into smaller sub-net
first address in the subnet = subnet address
last address in the subnet = subnet broadcast address
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19. 4.3.4 IP Sub-networking (Subnetting)
a single IP class A, B or C network is further divided into a group of hosts to form a sub-network
Advantages :
manageability
performance
security of hosts
reduce network congestion
subnet IP address
IP address that becomes the sub-network address (subnet address) of that sub-network
subnet broadcast IP address
IP address that is use to broadcast to all hosts in the sub-network
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20. 4.3.4 IP Sub-networking
Ex : For a given IP address and a subnet mask , answer the following.
Pre-question : change the address to slash notation
i) Determine the subnet IP address.
ii) Determine the broadcast IP address.
iii) Determine the valid address range of hosts IP addresses.
iv) Determine the number of valid hosts for the subnet
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21. 4.3.4 IP Sub-networking
Ex : Suppose an organization is given the block /26, which contains 64 addresses. The organizations has 3 offices and needs to divide the addresses into 3 sub-blocks (sub-network) of 32, 16 and 16 addresses. Find the new mask of every sub-network
Answer :
1. the first subnet has 32 addresses, then 232-n must be 32, means n = 27
2. the second subnet has 16 addresses.....
3. the third subnet....
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22. 4.3.4 IP Sub-networking Ex : example of subnetting 17.12.40.0/26
Find each host subnet address !
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23. 4.3.4 IP Sub-networking
Ex : An ISP is granted a block of addresses starting with /16 (65,536 addresses). The ISP needs to distribute these addresses to 3 groups as follows :
1st group has 64 customers, each needs 256 addresses
2nd group has 128 customers, each needs 128 addresses
3rd group has 128 customers, each needs 64 address
Design the subblocks and find how many addresses are still available after all these allocation.
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24. 4.3.5 Network Address Translation
increasing number of home users/small businesses using the internet
an ISP with a block of address could dynamically assign an address to these users for a specific period of time
but currently most of the home users/small businesses need more than one address as many have created a small network with several hosts where every hosts need an IP address
lead to a shortage of address problem
as a solution – Network Address Translation (NAT)
NAT enables users to have a large set of address internally and one address (or a small set of addresses) externally
traffic inside use the internal large set, while traffic outside use external small set
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25. 4.3.5 Network Address Translation
to separate the addresses used inside the home or business and the one used for the Internet, the internet Authorities have reserved 3 sets of address as private addresses :
any organization can use an address out of this set without permission from the Internet authorities
unique inside the organization, but not unique globally
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26. 4.3.5 Network Address Translation
the site must have only one single connection to the global Internet through a router to run the NAT software
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27. 4.3.5 Network Address Translation
all the outgoing packets go through the NAT router, which replaces the source address in the packet with the global NAT address.
all the incoming packets also pass through the NAT router, which replaces the destination address in the packet with the appropriate private address
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28. 4.3.5 Network Address Translation
Translation Table
NAT router has a translation table to forward the packets to the private addresses in the network
1. Using one IP address
2. Using a Pool of IP address
3. Using both IP addresses and port numbers
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29. 4.3.5 Network Address Translation
Using one IP address
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30. 4.3.5 Network Address Translation
Using a Pool of IP Addresses
since the NAT router has only one global address, only one private network host can access the same external host
to remove this restriction, the NAT router uses a poll of global address
ex : instead of using only one global address ( ), the NAT router can use 4 addresses ( , , and )
4 private hosts can connect with the same eternal hosts at the same time
no private network host can access two external server programs (HTTP and FTP) at the same time
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31. 4.3.5 Network Address Translation
Using both IP Addresses and port numbers
allow a many-to-many relationship between private-network hosts and external server programs, port number is included at the translation table
ex : suppose two hosts with addresses and inside the private network need to access the HTTP server on external host
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32. 4.3.5 Network Address Translation
NAT and ISP
ISP that serves dial-up customers can use NAT technology to conserve addresses
ex : suppose an ISP is granted 1000 addresses, but has 100,000 customers.
the ISP translates each of the 100,000 private addresses to one of the 1000 global addresses in the outgoing packets
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