Transcript IP addressing

Recap: “Operational” view of Internet
 Internet: “network of
Mobile network
networks”

Requires sending, receiving
of messages
 protocols control sending,
Global ISP
Home network
Regional ISP
receiving of messages

e.g., TCP, IP, HTTP, Skype,
Ethernet etc.
Institutional network
 Design of protocols is the
key for Internet
1-1 Lectu
re 1
Internet protocol stack
 application


support host/network applications
Email, FTP, HTTP (HTML)
 transport


process-process data transfer
TCP, UDP
 network


routing of datagrams from src. to destn.
IP address, routing protocols
 link


data transfer between neighboring network
elements
Ethernet, PPP
application
transport
network
link
physical
 physical

bits “on the wire”
1-2 Lectu
re 3
Network layer
 Network layer protocols
in every host, router
application
transport
network
data link
physical
network
data link
physical
 Router examines header
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
network
data link
data link
physical
physical
network
data link
physical
fields in all IP datagrams
passing through it
network
data link
physical
network
data link
physical
network
data link
physical
Network Layer
application
transport
network
data link
physical
4-3
Key Network-Layer Functions
 Forwarding: move
packets from router’s
input to appropriate
router output
 Routing: determine
route taken by
packets from source
to dest.
analogy:
 forwarding: process of
getting through single
interchange
 routing: process of
planning trip from source
to dest
Network Layer
4-4
Key Network-Layer Function
requirement:
 Need to know the
addresses
 Zip codes ~ e.g., 10019
 What is the address for
computers / routers?
Network Layer
4-5
IP Address
Network Layer
4-6
IP Address
 An IP address is a 32-bit sequence of 1s and 0s.
 To make the IP address easier to use, the address is
usually written as four decimal numbers separated by
periods.
 This way of writing the address is called the dotted
decimal format.
11011111 00000001 00000001 00000001
223
1
1
1
7
IP Addressing example network
 IP address: 32-bit
identifier for host,
router interface
 interface: connection
between host/router
and physical link



router’s typically have
multiple interfaces
host typically has one
interface
IP addresses
associated with each
interface
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.2
223.1.3.1
223.1.1.1 = 11011111 00000001 00000001 00000001
223
1
1
Network Layer
1
4-8
A quick look at Binary and
Decimal Number format
9
Decimal (base 10)
 Uses positional representation
 Each digit corresponds to a power of 10
based on its position in the number
 The powers of 10 increment from 0, 1, 2,
etc. as you move right to left
1,234 = 1 * 103 + 2 * 102 + 3 * 101 + 4 * 100
Binary (base 2)
 Two digits: 0, 1
 To make the binary numbers more
readable, the digits are often put in groups
of 4 or 8
1010 = 1 * 23 + 0 * 22 + 1 * 21 + 0 * 20
=8+2
= 10
1100 1001 = 1 * 27 + 1 * 26 + 1 * 23 + 1 * 20
= 128 + 64 + 8 + 1
= 201
Conversion
 From binary to decimal
 Use positional representation as shown in last slide
 From decimal to binary (tricky!)
 Keep dividing by 2
 Remainders give the digits, starting from lowest
power
 Let’s look at some examples…
 Now we are ready for IP addressing
IP address Class Handouts…
(provided in class)
Network Layer 4-13
IP Address
Every IP address has two parts:
1. Network part
2. Host part
IP addresses are divided into
classes A,B and C to define
-- large,
-- medium, and
-- small networks.
The Class D address class
was created to enable
multicasting.
Class E addresses reserved for future and research.
IP Address classes
Some special IP addresses
Network Layer 4-16
DHCP: Dynamic Host Configuration Protocol
Goal: allow host to dynamically obtain its IP address from
network server when it joins network
Can renew its lease on address in use
Allows reuse of addresses (only hold address while connected an
“on”)
Support for mobile users who want to join network (more shortly)
DHCP overview:
 host broadcasts “DHCP discover” msg [optional]
 DHCP server responds with “DHCP offer” msg
[optional]
 host requests IP address: “DHCP request” msg
 DHCP server sends address: “DHCP ack” msg
Network Layer 4-17
DHCP client-server scenario
A
B
223.1.2.1
DHCP
server
223.1.1.1
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
223.1.3.2
E
arriving DHCP
client needs
address in this
network
Network Layer 4-18
DHCP client-server scenario
DHCP server: 223.1.2.5
DHCP discover
arriving
client
src : 0.0.0.0, 68
dest.: 255.255.255.255,67
yiaddr: 0.0.0.0
transaction ID: 654
DHCP offer
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 654
Lifetime: 3600 secs
DHCP request
time
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
IP: 223.1.2.4
Network Layer 4-19
Numerical example
 A software company has 100 employees.



What would be the ideal class from which the company would choose its
network IP to prevent wastage of IP addresses?
How many bits would be assigned for network part and
how many bits would be assigned for host part?
 The company suddenly goes through increase in number of
employees from 100 to 2040.



What would be the ideal class from which the company would choose its
network IP to prevent wastage of IP addresses?
How many bits would be assigned for network part and
how many bits would be assigned for host part?
 Solve!
Network Layer 4-20
IP addressing: CIDR
CIDR: Classless InterDomain Routing
subnet portion of address of arbitrary length
 address format: a.b.c.d/x, where x is # bits in
subnet portion of address

 Back to the previous numerical example? How
many address wastage?
subnet
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/21
Network Layer 4-21
Network Address Translation
(NAT)
Network Layer 4-22
Home network
rest of
Internet
local network
(e.g., home network)
NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
10.0.0.4
10.0.0.1
10.0.0.2
138.76.29.7
10.0.0.3
All datagrams leaving local
network have same single source
NAT IP address: 138.76.29.7,
different source port numbers
Datagrams with source or
destination in this network
have 10.0.0/24 address for
source, destination (as usual)
NAT: Network Address Translation
 Advantages:


local network uses just one IP address as far as
outside world is concerned: min. IP address wastage
can change addresses of devices in local network
without notifying outside world: flexibility
 devices
inside local net not explicitly addressable,
visible by outside world (a security plus).
NAT: Network Address Translation
2: NAT router
changes datagram
source addr from
10.0.0.1, 3345 to
138.76.29.7, 5001,
updates table
2
NAT translation table
WAN side addr
LAN side addr
1: host 10.0.0.1
sends datagram to
128.119.40.186, 80
138.76.29.7, 5001 10.0.0.1, 3345
……
……
S: 10.0.0.1, 3345
D: 128.119.40.186, 80
S: 138.76.29.7, 5001
D: 128.119.40.186, 80
138.76.29.7
S: 128.119.40.186, 80
D: 138.76.29.7, 5001
3: Reply arrives
dest. address:
138.76.29.7, 5001
3
1
10.0.0.4
S: 128.119.40.186, 80
D: 10.0.0.1, 3345
10.0.0.1
10.0.0.2
4
10.0.0.3
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 10.0.0.1, 3345
NAT: Network Address Translation
 16-bit port-number field:

60,000 simultaneous connections with a single
LAN-side address!
NAT traversal problem
 client wants to connect to
server with address 10.0.0.1


server address 10.0.0.1 local
Client
to LAN (client can’t use it as
destination addr)
only one externally visible
NATted address: 138.76.29.7
?
138.76.29.7
 solution 1: statically
configure NAT to forward
incoming connection
requests at given port to
server
10.0.0.1
10.0.0.4
NAT
router
NAT traversal problem
 solution 2: relaying (used in Skype)
NATed client establishes connection to relay
 External client connects to relay
 relay bridges packets between connections

2. connection to
relay initiated
by client
Client
3. relaying
established
1. connection to
relay initiated
by NATted host
138.76.29.7
NAT
router
10.0.0.1