IP Address

0network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

32 bits

7 bits

14 bits

21 bits

28 bits

Class-based address:

IP addressing: CIDR• Classful addressing:

– inefficient use of address space, address space exhaustion– e.g., class B net allocated enough addresses for 65K hosts,

even if only 500 hosts in that network

• CIDR: Classless InterDomain Routing– network portion of address of arbitrary length– address format: a.b.c.d/x, where x is # bits in network portion

of address

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23

CIDR

• Network address: 200.23.16.0/23– /23 : network mask

• More efficient use of address– Consider a network with 500 hosts– Classful address: a class B address, wasting over 64K

addresses– CIDR: a network with /23– One class B address can be used for 128 such networks using

CIDR• Routing difficulty

– Classful: only need the IP address to determine the network add– CIDR: also need network mask information to determine the

network address– Longest match first

IP addresses: how to get one?Q: How does host get IP address?• hard-coded by system admin in a file

– Wintel: control-panel->network->configuration->tcp/ip->properties– UNIX: /etc/rc.config

• DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server– “plug-and-play” : host automatically obtain an IP and related info– mobile computing

IP addresses: how to get one?

Q: How does network get network part of IP addr?

A: gets allocated portion of its provider ISP’s address space (a good example for CIDR)

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

IP routing• IP address: 32-bit

identifier for host, router interface

• interface: connection between host/router and physical link– router’s typically have

multiple interfaces– host may have multiple

interfaces– IP addresses

associated with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

Network address: 223.1.1.0/24

IP Routing• IP address:

– network part (high order bits) is used for routing

– host part (low order bits) not used for routing

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 IP networks(for IP addresses starting with 223, first 24 bits are network address)

LAN

Getting a datagram from source to dest.

IP datagram:

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

miscfields

sourceIP addr

destIP addr data

• datagram remains unchanged, as it travels source to destination

• addr fields of interest here• Default router for all other networks

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2Others 223.1.1.4 x

forwarding table in A

Getting a datagram from source to dest.

Starting at A, send IP datagram addressed to B:

• look up net. address of B in forwarding table

• find B is on same net. as A• link layer will send datagram directly

to B inside link-layer frame– B and A are directly connected

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

miscfields223.1.1.1223.1.1.3data

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

forwarding table in A

Getting a datagram from source to dest.

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2Starting at A, dest. E:

• look up network address of E in forwarding table

• E on different network

– A, E not directly attached

• routing table: next hop router to E is 223.1.1.4

• link layer sends datagram to router 223.1.1.4 inside link-layer frame

• datagram arrives at 223.1.1.4

• continued…..

miscfields223.1.1.1223.1.2.3 data

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

forwarding table in A

Getting a datagram from source to dest.

Arriving at 223.1.4, destined for 223.1.2.2

• look up network address of E in router’s forwarding table

• E on same network as router’s

interface 223.1.2.9 – router, E directly attached

• link layer sends datagram to 223.1.2.2 inside link-layer frame

via interface 223.1.2.9 • datagram arrives at 223.1.2.2!!!

(hooray!)

miscfields223.1.1.1223.1.2.3 data Dest. Net router Nhops interface

223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9

223.1.3 - 1 223.1.3.27

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

forwarding table in router

CIDR Routing

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23

11001000 00010111 00000000 00000000

networkpart

hostpart

200.23.0.0/17

CIDR routing: longest match first

IP Fragmentation & Reassembly• network links have MTU

(max.transfer size) - largest possible link-level frame.– different link types,

different MTUs • large IP datagram divided

(“fragmented”) within net– one datagram becomes

several datagrams– “reassembled” only at

final destination– IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=1480

fragflag=1

length=1500

ID=x

offset=2960

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example• 4000 byte

datagram• MTU = 1500 bytes

LAN Addresses and ARP

32-bit IP address: • network-layer address• used to get datagram to destination IP network (recall IP

network definition)

LAN (or MAC or physical or Ethernet) address: • used to get datagram from one interface to another

physically-connected interface (same network)• 48 bit MAC address (for most LANs)

burned in the adapter ROM

LAN Addresses and ARPEach adapter on LAN has unique LAN address

LAN Address (more)

• MAC address allocation administered by IEEE• manufacturer buys portion of MAC address space (to

assure uniqueness)• Analogy:

(a) MAC address: like Social Security Number

(b) IP address: like postal address• MAC flat address => portability

– can move LAN card from one LAN to another

• IP hierarchical address NOT portable– depends on IP network to which node is attached

Recall earlier routing discussion

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

Starting at A, given IP datagram addressed to B:

• look up net. address of B, find B on same net. as A

• link layer send datagram to B

inside link-layer frame

B’s MACaddr

A’s MACaddr

A’s IPaddr

B’s IPaddr

IP payload

datagramframe

frame source,dest address

datagram source,dest address

ARP: Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP table

• ARP Table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; TTL>

– TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

Question: how to determineMAC address of Bknowing B’s IP address?

ARP protocol

• A wants to send datagram to B, and A knows B’s IP address.

• Suppose B’s MAC address is not in A’s ARP table.

• A broadcasts ARP query packet, containing B's IP address – all machines on LAN

receive ARP query

• B receives ARP packet, replies to A with its (B's) MAC address– frame sent to A’s MAC

address (unicast)

• A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) – soft state: information that

times out (goes away) unless refreshed

• ARP is “plug-and-play”:– nodes create their ARP

tables without intervention from net administrator

Routing to another LANwalkthrough: send datagram from A to B via R

assume A know’s B IP address

• Two ARP tables in router R, one for each IP network (LAN)

• In routing table at source Host, find router 111.111.111.110• In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc

A

RB

• A creates datagram with source A, destination B • A uses ARP to get R’s MAC address for 111.111.111.110

• A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram

• A’s data link layer sends frame • R’s data link layer receives frame • R removes IP datagram from Ethernet frame, sees its destined

to B• R uses ARP to get B’s physical layer address • R creates frame containing A-to-B IP datagram sends to B

A

RB

Ethernet

“dominant” LAN technology: • cheap $20 for 100Mbs!• first widely used LAN technology• Simpler, cheaper than token LANs and ATM

• Kept up with speed race: 10, 100, 1000 Mbps

Metcalfe’s Ethernetsketch

Ethernet Frame StructureSending adapter encapsulates IP datagram (or other

network layer protocol packet) in Ethernet frame

Preamble: • 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011• used to synchronize receiver, sender clock rates

Ethernet Frame Structure (more)

• Addresses: 6 bytes– if adapter receives frame with matching destination address, or

with broadcast address (eg ARP packet), it passes data in frame to net-layer protocol

– otherwise, adapter discards frame

• Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk)

• CRC: checked at receiver, if error is detected, the frame is simply dropped