Network Layer

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Network Layer PART IV

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Position of network layer

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Network layer duties

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Chapters Chapter 19 Host-to-Host Delivery Chapter 20 Network Layer Protocols Chapter 21 Unicast and Multicast Routing Protocols

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Chapter 19 Host-to-HostDelivery: Internetworking, Addressing, and Routing

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Figure Protocols at network layer

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IP Datagram Fragmentation

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Figure IP datagram

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The total length field defines the total length of the datagram including the header. Note:

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Addressing Internet Address Classful Addressing Supernetting Subnetting Classless Addressing Dynamic Address Configuration Network Address Translation

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An IP address is a 32-bit address. Note:

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The IP addresses are unique and universal. Note:

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Figure Dotted-decimal notation

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The binary, decimal, and hexadecimal number systems are reviewed in Appendix B. Note:

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Example 1 Change the following IP addresses from binary notation to dotted-decimal notation. a. 10000001 00001011 00001011 11101111 b. 11111001 10011011 11111011 00001111 Solution We replace each group of 8 bits with its equivalent decimal number (see Appendix B) and add dots for separation: a. 129.11.11.239 b. 249.155.251.15

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Example 2 Change the following IP addresses from dotted-decimal notation to binary notation. a. 111.56.45.78 b. 75.45.34.78 Solution We replace each decimal number with its binary equivalent (see Appendix B): a. 01101111 00111000 00101101 01001110 b. 01001011 00101101 00100010 01001110

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In classful addressing, the address space is divided into five classes: A, B, C, D, and E. Note:

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Figure Finding the class in binary notation

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Figure Finding the address class

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Example 3 Find the class of each address: a. 00000001 00001011 00001011 11101111 b. 11110011 10011011 11111011 00001111 Solution See the procedure in Figure 19.11. a. The first bit is 0; this is a class A address. b. The first 4 bits are 1s; this is a class E address.

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Figure Finding the class in decimal notation

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Example 4 Find the class of each address: a. 227.12.14.87 b. 252.5.15.111 c. 134.11.78.56 Solution a. The first byte is 227 (between 224 and 239); the class is D. b. The first byte is 252 (between 240 and 255); the class is E. c. The first byte is 134 (between 128 and 191); the class is B.

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Figure Netid and hostid

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Figure Blocks in class A

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Millions of class A addresses are wasted. Note:

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Figure Blocks in class B

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Many class B addresses are wasted. Note:

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The number of addresses in class C is smaller than the needs of most organizations. Note:

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Figure Blocks in class C

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In classful addressing, a large part of the available addresses are wasted. Note:

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Figure Network address In classful addressing, an IP address in class A, B or C is divided into netid and hostid 1st byte – Netid Next 3 bytes - Hostid 1st 2 bytes – Netid Next 2 bytes - Hostid 1st 3 bytes – Netid Next 1 byte - Hostid

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In classful addressing, the network address is the one that is assigned to the organization. Note:

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A network address is different from a netid. A network address has both netid and hostid, with 0s for the hostid. Note:

Mask : 

Mask A mask is a 32 bit number made of contiguous 1s followed by contiguous 0s The default masks for classes A,B and C are as shown in the table The mask can help in finding netid and hostid Note: No mask concept in Class D & E

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The network address can be foundby applying the default mask to anyaddress in the block (including itself).It retains the netid of the block and sets the hostid to 0s. Note:

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Table 19.1 Default masks

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Figure Sample internet

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Example 5 Given the address 23.56.7.91, find the network address. Solution The class is A. Only the first byte defines the netid. We can find the network address by replacing the hostid bytes (56.7.91) with 0s. Therefore, the network address is 23.0.0.0.

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Example 6 Given the address 132.6.17.85, find the network address. Solution The class is B. The first 2 bytes defines the netid. We can find the network address by replacing the hostid bytes (17.85) with 0s. Therefore, the network address is 132.6.0.0.

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Example 7 Given the network address 17.0.0.0, find the class. Solution The class is A because the netid is only 1 byte.

Subnetting : 

Subnetting If an organization was granted a large block in class A or B, it can divide the addresses into several contiguous groups and assign each group to smaller networks

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Figure Addresses in a network with and without Subnetting If an organization was granted a large block in class A or B, it could divide the addresses into several contiguous groups and assign each group to smaller networks-subnet.

Supernetting : 

Supernetting Due to additional demand of midsize blocks Class A & Class B addresses were depleted & size of Class C block did not satisfy the needs of most organizations One Solution – Supernetting –Several Class C blocks are combined to create a larger range of addresses i.e Several networks are combined to create a supernet.

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Subnetting increases the number of 1s in the mask Supernetting decreases the number of 1s in the mask Note:

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Example 8 A router outside the organization receives a packet with destination address 190.240.7.91. Show how it finds the network address to route the packet. Solution The router follows three steps: The router looks at the first byte of the address to find the class. It is class B. The default mask for class B is 255.255.0.0. The router ANDs this mask with the address to get 190.240.0.0. The router looks in its routing table to find out how to route the packet to this destination. Later, we will see what happens if this destination does not exist.

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Figure Subnet mask

Problems with Classful addressing : 

Problems with Classful addressing Fast growth of Internet led to near depletion of available addresses We have run out of Class A & Class B addresses Class C address block is too small for most mid size organizations Hence Classful addressing is replaced with Classless addressing

Classless Addressing : 

Classless Addressing In this scheme, there are no classes but the addresses are still granted in blocks Address Blocks When an entity needs to be connected to the Internet it is granted a block of addresses The size of the block varies based on the nature and size of the entity Ex: A household- 2 addresses An ISP – 1000’s or 100’s of addresses

Restrictions : 

Restrictions To simplify the handling of addresses, the Internet authorities impose three restrictions on classless address blocks: The addresses in a block must be contiguous, one after another The number of addresses in a block must be a power of 2 The first address must be evenly divisible by the by number of addresses.

Mask-Classless addressing : 

Mask-Classless addressing An IPv4 addressing, a block of addresses can be defined as x.y.z.t/n x.y.z.t – defines one of the addresses /n – mask Ex.123.34.45.2/24 - indicates that the first 3 bytes are the network address & last byte is the host address.

First Address : 

First Address The first address in the block can be found by setting the rightmost 32-n bits to 0s Last Address The last address in the block can be found by setting the rightmost 32-n bits to 1s Number of Addresses The number of addresses in the block can be found by using the formula 232-n

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A network address is different from a netid. A network address has both netid and hostid, with 0s for the hostid. Note: Network Address

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The first address in a block is normally not assigned to any device; it is used as the network address that represents the organization to the rest of the world. Note:

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IP addresses are designed with two levels of hierarchy. Note:

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Figure Hierarchy concept in a telephone number

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Figure A network with two levels of hierarchy

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Figure A network with three levels of hierarchy (subnetted)

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Example 9 A router inside the organization receives a packet with destination address 190.240.33.91/19. Show how it finds the subnetwork address to route the packet. Solution The router follows three steps: The router must know the mask. It is /19, as given in the destn. Address. The router applies the mask to the address, 190.240.33.91. The subnet address is 190.240.32.0. The router looks in its routing table to find how to route the packet to this destination.

NAT : 

NAT The number of users and small businesses that want to use the Internet is ever increasing An address was given to a user when it was needed dynamically But this is not possible due to shortage of addresses Hence a simple solution is – Network Address Translation

NAT : 

NAT Enables a user to have a large set of addresses internally & a small set of addresses externally To separate these addresses the Internet authorities have reserved three sets of addresses as private addresses as shown in table. These addresses are unique inside the orgn. But are not unique globally No router will forward a packet that has one of these addresses as the destination address

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Table 19.2 Default masks

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Figure NAT Implementation

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Figure Address translation

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Figure NAT Address Translation

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Table 19.3 Five-column translation table

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IPv6 IPv6 Addresses Categories of Addresses IPv6 Packet Format Fragmentation ICMPv6 Transition

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Figure IPv6 address

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Figure Abbreviated address

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Figure Abbreviated address with consecutive zeros

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Figure CIDR address

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Figure Format of an IPv6 datagram

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Figure Comparison of network layers in version 4 and version 6

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Figure Three transition strategies

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Figure Three transition strategies

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Figure Tunneling

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Figure Header translation