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COS 420 . Day 3. Agenda. Assignment Due Jan 29, 2003 Next Class Individual Projects assigned Today. Individualized project. Will be a research project & paper ~ 20 page paper MLA Format 10 Min Presentation You can pick any topic that one of the IETF working groups is developing

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COS 420

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COS 420

Day 3


  • Assignment Due Jan 29, 2003

    • Next Class

  • Individual Projects assigned Today

Individualized project

  • Will be a research project & paper

    • ~ 20 page paper

      • MLA Format

    • 10 Min Presentation

  • You can pick any topic that one of the IETF working groups is developing


    • Past history, Current state, upcoming developments

  • Due Date

    • Papers due March 15

    • Presentations will be on March 22


  • Definitions

    • Name

      • Identifies what an entity is

      • Often textual (e.g., ASCII)

    • Address

      • Identifies where an entity is located

      • Often binary and usually compact

      • Sometimes called locator

    • Route

      • Identifies how to get to the object

      • May be distributed

Internet Protocol Address(IP Address)

  • Analogous to hardware address

  • Unique value assigned as unicast address to each host on Internet

  • Used by Internet applications

IP Address Details

  • 32-bit binary value

  • Unique value assigned to each host in Internet

  • Values chosen to make routing efficient

IP Address Division

  • Address divided into two parts

    • Prefix (network ID) identifies network to which host attaches

    • Suffix (host ID) identifies host on that network

Classful Addressing

  • Original IP scheme

  • Explains many design decisions

  • New schemes are backward compatible

Desirable Properties Of AnInternet Addressing Scheme

  • Compact (as small as possible)

  • Universal (big enough)

  • Works with all network hardware

  • Supports efficient decision making

    • Test whether a destination can be reached directly

    • Decide which router to use for indirect delivery

    • Choose next router along a path to the destination

Division Of Internet AddressInto Prefix And Suffix

  • How should division be made?

    • Large prefix, small suffix means many possible networks, but each is limited in size

    • Large suffix, small prefix means each network can be large, but there can only be a few networks

  • Original Internet address scheme designed to accommodate both possibilities

    • Known as classful addressing

Original IPv4 Address Classes

Important Property

  • Classful addresses are self-identifying

  • Consequences

    • Can determine boundary between prefix and suffix from the address itself

    • No additional state needed to store boundary information

    • Both hosts and routers benefit

Endpoint Identification

  • Because IP addresses encode both a network and a host on that network, they do not specify an individual computer, but a connection to a network.

IP Address Conventions

  • When used to refer to a network

    • Host field contains all 0 bits

  • Broadcast on the local wire

    • Network and host fields both contain all 1 bits

  • Directed broadcast: broadcast on specific (possibly remote) network

    • Host field contains all 1 bits

    • Nonstandard form: host field contains all 0 bits

Assignment Of IP Addresses

  • All hosts on same network assigned same address prefix

    • Prefixes assigned by central authority

    • Obtained from ISP

  • Each host on a network has a unique suffix

    • Assigned locally

    • Local administrator must ensure uniqueness

Advantages Of Classful Addressing

  • Computationally efficient

    • First bits specify size of prefix / suffix

  • Allows mixtures of large and small networks

Directed Broadcast

  • IP addresses can be used to specify a directed broadcast in which a packet is sent to all computers on a network; such addresses map to hardware broadcast, if available. By convention, a directed broadcast address has a valid netid and has a hostid with all bits set to 1.

Limited Broadcast

  • All 1’s

  • Broadcast limited to local network only (no forwarding)

  • Useful for bootstrapping

All Zeros IP Address

  • Can only appear as source address

  • Used during bootstrap before computer knows its address

  • Means ‘‘this’’ computer

Internet Multicast

  • IP allows Internet multicast, but no Internet-wide multicast delivery system currently in place

  • Class D addresses reserved for multicast

  • Each address corresponds to group of participating computers

  • IP multicast uses hardware multicast when available

  • More later in the course

Consequences Of IP Addressing

  • If a host computer moves from one network to another, its IP address must change

  • For a multi-homed host (with two or more addresses), the path taken by packets depends on the address used

Multi-Homed Hosts And Reliability

  • Knowing that B is multi-homed increases reliability

Dotted Decimal Notation

  • Syntactic form for expressing 32-bit address

  • Used throughout the Internet and associated literature

  • Represents each octet in decimal separated by periods (dots)

Example Of Dotted DecimalNotation

  • A 32-bit number in binary

    • 10000000 00001010 00000010 00000011

  • The same 32-bit number expressed in dotted decimal notation

    • 128 . 10 . 2 . 3

Loopback Address

  • Used for testing

  • Refers to local computer (never sent to Internet)

  • Address is

Classful Address Ranges

Summary Of Address Conventions

Example Of IP Addressing

  • Assume an organization has three networks

  • Organization obtains three prefixes, one per network

  • Host address must begin with network prefix

Illustration Of IP Addressing


  • IP address

    • 32 bits long

    • Prefix identifies network

    • Suffix identifies host

  • Classful addressing uses first few bits of address to determine boundary between prefix and suffix Special forms of addresses handle

    • Limited broadcast

    • Directed broadcast

    • Network identification

    • This host

    • Loopback






  • Must use hardware (physical) addresses to communicate over network

  • Applications only use Internet addresses


  • Computers A and B on same network

  • Application on A generates packet for application on B

  • Protocol software on A must use B’s hardware address when sending a packet


  • Protocol software needs a mechanism that maps an IP address to equivalent hardware address

  • Known as address resolution problem

Address Resolution

  • Performed at each step along path through Internet

  • Two basic algorithms

    • Direct mapping

    • Dynamic binding

  • Choice depends on type of hardware

Direct Mapping

  • Easy to understand

  • Efficient

  • Only works when hardware address is small

  • Technique: assign computer an IP address that encodes the hardware address

Example Of Direct Mapping

  • Hardware: proNet ring network

  • Hardware address: 8 bits

  • Assume IP address (24-bit prefix)

  • Assign computer with hardware address K an IP address

  • 192.5.48.K

  • Resolving an IP address means extracting the hardware address from low-order 8 bits

Dynamic Binding

  • Needed when hardware addresses are large (e.g., Ethernet)

  • Allows computer A to find computer B’s hardware address

    • A starts with B’s IP address

    • A knows B is on the local network

  • Technique: broadcast query and obtain response

    Note: dynamic binding only used across one network at a time

Internet Address Resolution Protocol (ARP)

  • Standard for dynamic address resolution in the Internet

  • Requires hardware broadcast

  • Intended for LAN

  • Important idea: ARP only used to map addresses within a single physical network, never across multiple networks


  • Machine A broadcasts ARP request with B’s IP address

  • All machines on local net receive broadcast

  • Machine B replies with its physical address

  • Machine A adds B’s address information to its table

  • Machine A delivers packet directly to B

Illustration Of ARPRequest And Reply Messages

ARP Packet Format WhenUsed With Ethernet

Observations About Packet Format

  • General: can be used with

    • Arbitrary hardware address

    • Arbitrary protocol address (not just IP)

  • Variable length fields (depends on type of addresses)

  • Length fields allow parsing of packet by computer that does not understand the two address types

Retention Of Bindings

  • Cannot afford to send ARP request for each packet

  • Solution

    • Maintain a table of bindings

  • Effect

    • Use ARP one time, place results in table, and then send many packets

ARP Caching

  • ARP table is a cache

  • Entries time out and are removed

  • Avoids stale bindings

  • Typical timeout: 20 minutes

Algorithm For ProcessingARP Requests

  • Extract sender’s pair, (IA, EA) and update local ARP table if it exists

  • If this is a request and the target is ‘‘me’’

    • Add sender’s pair to ARP table if not present

    • Fill in target hardware address

    • Exchange sender and target entries

    • Set operation to reply

    • Send reply back to requester

Algorithm Features

  • If A ARPs B, B keeps A’s information

    • B will probably send a packet to A soon

  • If A ARPs B, other machines do not keep A’s information

    • Avoids clogging ARP caches needlessly

Conceptual Purpose Of ARP

  • Isolates hardware address at low level

  • Allows application programs to use IP addresses

ARP Encapsulation

  • ARP message travels in data portion of network frame

  • We say ARP message is encapsulated

Illustration Of ARP Encapsulation

Ethernet Encapsulation

  • ARP message placed in frame data area

  • Data area padded with zeroes if ARP message is shorter

  • than minimum Ethernet frame

  • Ethernet type 0x0806 used for ARP


  • Computer’s IP address independent of computer’s hardware address

  • Applications use IP addresses

  • Hardware only understands hardware addresses

  • Must map from IP address to hardware address fortransmission

  • Two types

    • Direct mapping

    • Dynamic mapping

  • Address Resolution Protocol (ARP) used for dynamic address mapping

  • Important for Ethernet

  • Sender broadcasts ARP request, and target sends ARP reply

  • ARP bindings are cached




IP Address Assignment

  • For conventional computer

    • IP address stored on disk

    • OS obtains address by reading from file at startup

  • For diskless computer

    • IP address obtained from server

Reverse Address ResolutionProtocol (RARP)

  • Old protocol

  • Designed for diskless computer

  • Obtains an IP address

  • Adapted from ARP

  • Broadcasts request to server

  • Waits for response

Ethernet Encapsulation

  • RARP message carried in data portion of Ethernet frame

  • Ethernet type 0x0835 assigned to RARP

Illustration Of Packet Flow

  • In (a) client broadcasts a request

  • In (b) one or more servers respond

Client Identification

  • Computer must identify itself

  • RARP uses network hardware address as unique ID

    • Only works on network with permanent address (e.g., Ethernet)

Modern Bootstrap

  • Except for a few special cases, RARP has largely been replaced by DHCP

  • We will postpone further discussion of bootstrapping until later in the course when we can consider DHCP

For next week

  • Assignment #2 will be posted

  • We begin a more in depth look at IP

    • IP Architecture

    • IP routing

    • IP error and control messaging

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