cmpe 150 fall 2005 lecture 25

1CMPE 150- Introduction to Computer Networks

CMPE 150

Fall 2005Lecture 25

Introduction to Computer Networks

2CMPE 150- Introduction to Computer Networks

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!

3CMPE 150- Introduction to Computer Networks

Today

• IP (Cont’d).

4CMPE 150- Introduction to Computer Networks

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.

5CMPE 150- Introduction to Computer Networks

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).

6CMPE 150- Introduction to Computer Networks

IP Datagram Format

• IP datagram consists of header and data (or payload).

• Header:– 20-byte fixed (mandatory) part.

– Variable length optional part.

7CMPE 150- Introduction to Computer Networks

The IP v4 Header

8CMPE 150- Introduction to Computer Networks

IP Options

5-54

9CMPE 150- Introduction to Computer Networks

IP Addresses

• IP address formats.

10CMPE 150- Introduction to Computer Networks

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.

11CMPE 150- Introduction to Computer Networks

IP Addresses (Cont’d)

• Special IP addresses.

12CMPE 150- Introduction to Computer Networks

Scalability of IP Addresses

• Problem: a single A, B, or C address refers to a single network.

• As organizations grow, what happens?

13CMPE 150- Introduction to Computer Networks

Example: A Campus Network

14CMPE 150- Introduction to Computer Networks

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.

15CMPE 150- Introduction to Computer Networks

Subnets

• A class B network subnetted into 64 subnets.

16CMPE 150- Introduction to Computer Networks

Subnet Mask

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

Subnet Mask: 255.255.252.0 or /22 (network + subnet part)

17CMPE 150- Introduction to Computer Networks

Subnetting: Observations

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

made by local network admin.

18CMPE 150- Introduction to Computer Networks

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

19CMPE 150- Introduction to Computer Networks

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!

20CMPE 150- Introduction to Computer Networks

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.

21CMPE 150- Introduction to Computer Networks

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.

22CMPE 150- Introduction to Computer Networks

NAT – Network Address Translation

23CMPE 150- Introduction to Computer Networks

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.

24CMPE 150- Introduction to Computer Networks

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.

25CMPE 150- Introduction to Computer Networks

ICMP Message Types

5-61

26CMPE 150- Introduction to Computer Networks

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).

27CMPE 150- Introduction to Computer Networks

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.

28CMPE 150- Introduction to Computer Networks

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.

29CMPE 150- Introduction to Computer Networks

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.

30CMPE 150- Introduction to Computer Networks

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.

31CMPE 150- Introduction to Computer Networks

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.

32CMPE 150- Introduction to Computer Networks

DHCP Operation

33CMPE 150- Introduction to Computer Networks

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.