Fall 2006 Computer Networks 19-1

Chapter 19. Host-to-Host Delivery:

Internetworking, Addressing, and Routing

19.1 Internetworks

19.2 Addressing

19.3 Routing

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Position of network layer

• The network layer is responsible for host-to-host delivery

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Network layer duties

• Other issues– Address resolution– Multicasting– Routing protocols

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Internetwork

• Data exchange between networks: internetwork

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Links in an internetwork

• Frame in data link layer does not carry any routing information

• Problem: How does S1 know that data should be sent out from interface f3 ?

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Network layer in an internetwork

• Network layer is responsible for host-to-host delivery and for routing the packets

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Network layer at the source

• Responsible for creating a packet that carries two universal addresses: destination and source addresses

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Network layer at a router

• Network layer at the switch or router is responsible for routing the packet.

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Network layer at the destination

• Responsible for address verification

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Switching

• Switching at the network layer in the Internet is done using the datagram approach to packet switching.

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Datagram approach

• Switching at the network layer in the Internet is done using the datagram approach to packet switching

• Advantages: no call setup and VC identifier

• Disadvantages: out of order

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Internet as a Connectionless Network

• Communication at the network layer in the Internet is connectionless

• Connection-oriented service: Virtual circuit approach to packet switching such as in Frame Relay and ATM

• Connectionless service: datagram approach

• In connectionless service, the network layer protocol treats each packet independently

• Why Internet has chosen connectionless ?– Internet is made of so many heterogeneous networks that is almost

impossible to create a connection from the source to the destination without knowing the nature of the networks in advance

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Addressing

• An IP address is a 32-bit address.

• The IP addresses are unique and universal.

• Dotted-decimal notation

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Example

• Change the following IP addresses from binary notation to dotted-decimal notation.

a. 10000001 00001011 00001011 11101111

b. 11111001 10011011 11111011 00001111

• We replace each group of 8 bits with its equivalent decimal number and add dots for separation:

a. 129.11.11.239

b. 249.155.251.15

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Classful addressing

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

• A new architecture, called classless addressing was introduced in the mid-1990s

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Finding the address class

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Class in decimal notation

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Example

• Find the class of each address:

a. 227.12.14.87

b. 252.5.15.111

c. 134.11.78.56

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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Netid and hostid

• IP address in classes A, B, and C is divided into netid and hostid

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Class and Blocks• Problem in classful addressing is that each class is divided into a fixed number of bl

ocks with each block having a fixed size• Millions of class A addresses are wasted.

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Blocks in class B

• Many class B addresses are wasted.

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Blocks in class C

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

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Network address

• In classful addressing, the network address is the one that is assigned to the organization.• A network address is different from a netid. A network address has both netid and hostid,

with 0s for the hostid.

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Sample Internet with classful addresses

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Subnetting (two levels of hierarchy)

• IP addresses are designed with two levels of hierarchy.

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Subnetting (three levels)

• A network is divided into several smaller groups with each subnetwork having its own subnetwork address

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Addresses in a network with and without subnetting

• Hierarchy concept in a telephone number

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Mask

• The network address can be found by applying the default mask to any address in the block (including itself).

• It retains the netid of the block and sets the hostid to 0s.

Class In Binary In Dotted-Decimal Using Slash

A11111111 00000000 00000000 00000000 255.0.0.0 /8

B11111111 11111111 00000000 00000000 255.255.0.0 /16

C 11111111 111111111 11111111 00000000 255.255.255.0 /24

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Subnet mask

• Combine serveral class C blocks to create a larger range of addresses

Supernetting

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Classless addressing

• Classful addressing has created many problems

• Many ISPs and service users need more addresses

• Idea is to have variable-length blocks that belong to no class

• CIDR (Classless InterDomain Routing) for classless addressing, instead of classful routing

• DHCP (Dynamic Host Configuration Protocol) for dynamic addressing– DHCP provides the following information dynamically (on demand)– IP address, subnet mask, IP address of a router, IP address of a name server

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DHCP• Client-server program• DHCP server has two databases, one binds statically physical addresses to IP addresses and

another makes DHCP dynamic by assigning available (unused) IP addresses for a negotiable period of time

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Network Address Translation: NAT

• NAT enables a user to have a large set of addresses internally and one address, or a small set of addresses, externally.

Range Total

10.0.0.0 to 10.255.255.255 224

172.16.0.0 to 172.31.255.255 220

192.168.0.0 to 192.168.255.255 216

Addresses for private networks

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Address translation

• Address translation for source address of outgoing packet and for destination address of incoming packet

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Translation table

• Using (1) one IP address, (2) a pool of IP address, and (3) both IP addresses and port numbers

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Five-column translation table

Private Address

Private Port

ExternalAddress

External Port

TransportProtocol

172.18.3.1 1400 25.8.3.2 80 TCP

172.18.3.2 1401 25.8.3.2 80 TCP

... ... ... ... ...

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Routing Techniques

• Routing requires a host or router to have a routing table

• When a router receives a packet to be forwarded, it looks at the routing table to find the route to the final destination

• Several techniques can make the size of the routing table manageable and handle issues such as security– Next-Hop Routing– Network-Specific Routing– Host-Specific Routing– Default Routing

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Next-hop routing

• To reduce the contents of a routing table• Routing table holds only the information that leads to the next hop instead of

holding information about the complete route

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Network-specific routing

• To reduce the routing table and simplify the searching process

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Host-specific routing• Inverse of network-specific routing• Destination host address is given in the routing table• Not efficient, but administrator has greater control over routing

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Default routing

• Another technique to simplify routing

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Static versus dynamic routing

• Static routing table– Information entered manually– Small internet that does not change very often

• Dynamic routing table– Updated periodically using one of the dynamic routing protocols such

as RIP, OSPF, BGP– Update all the tables in the routers upon changes in Internet

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Routing table for classful addressing

• Routing table needs a minimum of four columns (it normally has more)• When a packet arrives, the router applies the mask to the destination address to find the corresponding

destination network address. If found, the packet is sent out from the corresponding interface in the table. If not found, the packet is delivered to the default interface.

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Routing table for classless addressing (CIDR)

• Routing table size can be increased or decreased• Decrease if the block of addresses assigned to an organization is larger than the blo

ck in classful addressing• Mostly increase because the intent of classless addressing is to divide up the blocks

of class A and class B addresses.

• To solve the problem of gigantic routing table– Hierarchical routing– Geographical routing– Routing table searching algorithms