CMPE 150 Fall 2005 Lecture 25

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CMPE 150 Fall 2005Lecture 25

• Introduction to Computer Networks

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Announcements

• Homework 4 due today by midnight.
• No class on Friday, 11.25.05.
• IMPORTANT No lab tonight.
• Make up lab session next week.
• Final exam December 7th. 4-7pm
• In class.
• Closed books/notes.
• Course evaluation.
• Need volunteers!
• CE 151 will be offered in Winter 06!

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Today

• IP (Contd).

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IP (Internet Protocol)

• Glues Internet together.
• Common network-layer protocol spoken by all
  Internet participating networks.
• Best effort datagram service
• No reliability guarantees.
• No ordering guarantees.

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IP Versions

• IPv4 IP version 4.
• Current, predominant version.
• 32-bit long addresses.
• IPv6 IP version 6 (aka, IPng).
• Evolution of IPv4.
• Longer addresses (16-byte long).

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IP Datagram Format

• IP datagram consists of header and data (or
  payload).
• Header
• 20-byte fixed (mandatory) part.
• Variable length optional part.

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The IP v4 Header
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IP Options
5-54
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IP Addresses

• IP address formats.

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IP Addresses (Contd)

• Class A 128 networks with 16M hosts each.
• Class B 16,384 networks with 64K hosts each.
• Class C 2M networks with 256 hosts each.
• More than 500K networks connected to the
  Internet.
• Network numbers centrally administered by ICANN.

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IP Addresses (Contd)

• Special IP addresses.

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Scalability of IP Addresses

• Problem a single A, B, or C address refers to a
  single network.
• As organizations grow, what happens?

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Example A Campus Network
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Solution

• Subnetting divide the organizations address
  space into multiple subnets.
• How? Use part of the host number bits as the
  subnet number.
• Example Consider a university with 35
  departments.
• With a class B IP address, use 6-bit subnet
  number and 10-bit host number.
• This allows for up to 64 subnets each with 1024
  hosts.

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Subnets

• A class B network subnetted into 64 subnets.

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Subnet Mask

• Indicates the split between network and subnet
  number host number.

Subnet Mask 255.255.252.0 or
/22 (network subnet part)
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Subnetting Observations

• Subnets are not visible to the outside world.
• Thus, subnetting (and how) is a decision made by
  local network admin.

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Subnet Example

• Subnet 1 10000010 00110010 00000100 00000001
• 130.50.4.1
• Subnet 2 10000010 00110010 00001000 00000001
• 130.50.8.1
• Subnet 3 10000010 00110010 00001100 00000001
• 130.50.12.1

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Problem with IPv4

• IPv4 is running out of addresses.
• Problem class-based addressing scheme.
• Example Class B addresses allow 64K hosts.
• More than half of Class B networks have fewer
  than 50 hosts!

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Solution CIDR

• CIDR Classless Inter-Domain Routing.
• RFC 1519.
• Allocate remaining addresses in variable-sized
  blocks without considering classes.
• Example if an organization needs 2000 addresses,
  it gets 2048-address block.
• Forwarding had to be modified.
• Routing tables need an extra entry, a 32-bit
  mask, which is ANDed with the destination IP
  address.
• If there is a match, the packet is forwarded on
  that interface.

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Network Address Translation

• Another quick fix to the address shortage in IP
  v4.
• Specified in RFC 3022.
• Each organization gets a single (or small number
  of) IP addresses.
• This is used for Internet traffic only.
• For internal traffic, each host gets its own
  internal IP address.
• Three IP ranges have been declared as private.
• 10.0.0.0 10.255.255.255/8
• 172.16.0.0 172.31.255.255/12
• 192.168.0.0 192.168.255.255/16
• No private IP address can show up on the
  Internet, i.e., outside the organizations
  network.

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NAT Network Address Translation
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Internet Control Protocols

• Companion protocols to IP.
• Control protocols used mainly for signaling and
  exchange of control information.
• Examples ICMP, ARP, RARP, BOOTP, and DHCP.

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ICMP

• Internet Control Message Protocol.
• A way to debug the Internet and find out what
  is happening at routers.
• Defines a dozen different messages that are
  generated typically by routers upon some
  unexpected event.

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ICMP Message Types
5-61
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Address Resolution Protocol

• ARP.
• RFC 826.
• Protocol for machines to map IP addresses to
  Ethernet addresses.
• This is needed when packet needs to be delivered
  to a local host on a LAN (Ethernet).

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ARP Example
. Host 1 wants to send packet to host 2. .
Assume that host 1 knows host 2s IP address.
. Host 1 builds packet with host 2s IP address.
. IP knows its a local destination but now
needs host 2s Ethernet address.
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ARP Operation

• Host 1 broadcasts an ARP request on the Ethernet
  asking who owns host 2s IP address.
• Host 2 replies with its Ethernet address.
• Some optimizations
• ARP caches.
• Piggybacking hosts own Ethernet address on ARP
  requests.
• Proxy ARP services ARP requests for hosts on
  separate LANs.

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Beyond ARP

• ARP solves the problem of mapping IP address to
  Ethernet address.
• How do we solve the inverse problem?
• I.e., how to map an Ethernet address to an IP
  address?
• Older protocols RARP (RFC 903) and BOOTP (RFC
  951).
• RARP broadcasts not forwarded by routers.
• BOOTP uses UDP but requires manual configuration
  of IP-Ethernet mappings.

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DHCP

• Dynamic Host Configuration Protocol.
• RFCs 2131 and 2132.
• Assigns IP addresses to hosts dynamically.
• DHCP server may not be on the same LAN as
  requesting host.
• DHCP relay agent.

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DHCP Operation

• Newly booted host broadcasts a DHCP DISCOVER
  message.
• DHCP relay agent intercepts DHCP DISCOVERs on its
  LAN and unicasts them to DHCP server.

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DHCP Operation
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DHCP Address Reuse

• How long should an IP address be allocated?
• Issue hosts come and go.
• IP addresses may be assigned on a Lease basis.
• Hosts must renew their leases.