Transcript PPT

Part 3: Internetworking
Internet architecture, addressing,
encapsulation, reliable transport and
the TCP/IP protocol suite
Internetworking: concepts,
architecture and protocols
Motivation, architecture, routers,
TCP/IP protocols, internet reference
model
Motivation
• A large organisation will use several
networking technologies
• Inter-organisational communication is
significant
• Universal service - any two computers
should be able to communicate
• However, different network technologies
cannot just be wired together
Internetworking
• Interconnect heterogeneous networks and
provide universal service
– Hardware: routers connect different networks
– Internet protocols: provide universal service by
creating a single virtual network
Internet architecture
• Although a single router can connect many
networks, most organisations use multiple
routers
Virtual
network
• The illusion
that there is a
single
universal
network
Internetworking protocols
• The TCP/IP Internet Protocols
– begun in the 1970s
– The Internet has emerged into the public
domain in the 1990s
– Controlled by the Internet Engineering Task
Force (IETF)
Internet Reference Model
Host computers
• TCP/IP used the term host computer to
refer to any system that connects to an
Internet and that runs applications
• Both hosts and routers use TCP/IP protocol
software
IP: Internet protocol addresses
Uniform addressing, the IP address
hierarchy, address classes, dotted
decimal notation, special addresses,
routers and addresses, address
resolution
Uniform addressing
• Internet protocols deal in packets and provide
uniform addressing
• The addressing scheme is defined in software
and is used transparently by applications
• Internet addressing is specified in the IP
protocol
• Each host is assigned a unique 32 bit address
The IP address hierarchy
• Each 32 bit address is divided into two parts
– prefix: physical network to which the host is
attached - the network number
– suffix: a host attached to a given physical network
• Prefixes are coordinated globally and suffixes
locally
Classes of IP address
• Size of prefix and suffix determines maximum
number of networks and maximum number of
hosts per network
• IP defines different classes of address with
different sized prefixes and suffixes
• The first four bits of the address specify its
class
The five classes of IP address
Dotted decimal notation
• Makes it easier to for humans to use
addresses (names are also possible)
Classes and dotted decimal
Division of the address space
• Public Internet network numbers are
assigned by Internet Service providers (ISPs)
and these are coordinated by the Internet
Assigned Number Authority
An addressing example
Classless Addressing
• The Internet is running out of addresses
• Allow division between prefix and suffix to appear at
an arbitrary boundary
• Consider network with only 9 hosts
– Only need four bits for host suffix
– Class C (smallest) address uses 8 bits for host suffix
– Can subdivide a class C address into 16 addresses with a
28 bit prefix and 4 bit suffix
•
Extend dotted decimal notation
– 193.68.138.0/28, 193.68.138.16/28, …,193.68.138.240/28
Special IP addresses
Routers and IP addressing
• Routers are assigned
two or more IP
addresses
• So are multi-homed
computers
Binding protocol addresses
• An Internet packet passes through a series
of routers
– each hop takes it over a particular network,
either to a specific computer on that network
or to the next router
– in either case, the sending router has to map
between the protocol (IP) address and a
hardware address
– this is called address resolution
Address resolution techniques
• Table lookup
• Closed-form computation
• Message exchange
– send message to specific server computers
– broadcast message, only the required computer
responds
Pros and cons of techniques
Address resolution protocol
• TCP/IP defines the Address Resolution
Protocol (ARP) which defines the format of
resolution requests and responses
• This technique is usually combined with local
caching of hardware addresses
Dynamic Host Configuration
Protocol (DHCP)
• Special DHCP server that assigns IP addresses to
hosts
• Newly booted machine broadcasts a DHCP discover
packet
• DHCP server sends back an IP address
– Permanent IP addresses
• Manually assigned by administrator
– Automatic IP address from a pool of addresses to be
allocated on demand
• Leased for a finite period of time
DHCP Operation
• DHCP server does not need to be on the same
network as the host
Summary
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Uniform addressing
Address classes
Dotted decimal notation
Classless addressing
Special IP addresses
Address resolution (ARP) and ssignment (DHCP)