1CMPE 150- Introduction to Computer Networks

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 (Cont’d).

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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 (Cont’d)

• 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 (Cont’d)

• 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 organization’s 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 000001|00 00000001

– 130.50.4.1

• Subnet 2: 10000010 00110010 000010|00 00000001– 130.50.8.1

• Subnet 3: 10000010 00110010 000011|00 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 organization’s 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 2’s IP address. . Host 1 builds packet with host 2’s IP address. . IP knows it’s a local destination but now needs host 2’s Ethernet address.

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ARP Operation

• Host 1 broadcasts an ARP request on the Ethernet asking who owns host 2’s IP address.

• Host 2 replies with its Ethernet address.

• Some optimizations:– ARP caches.– Piggybacking host’s 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.