ccnet-lec-10-ip-nat[1]

8/6/2019 ccnet-lec-10-IP-NAT[1]

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Lecture

10Data Communication & Networks

Internet Protocol (IP)

Muhammad Yousaf


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Internet Protocol: (IP)

IP is a layer-3 protocolStandardized by IETF as RFC-791

Backbone of the Internet

Internet is the network of networks

Thousands of networks are interconnected with eachother via routers

Routers forward packets on the basis of IP

Thats why, routers are also termed as layer-3devices


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Network of Networks:

Router, Aninterconnecting

node

Network-3Network-2

Network-1


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Functionality of IP:

IP is a connectionless protocolProvides no guarantees, a best effort protocol

It performs the following important communicationtasks:

Global Addressing

Fragmentation & Reassembly

Packet Forwarding


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TCP / IP protocol stack:

Source Router-1 Router-2 Destination

Physical

Data Link

IP

TCP | UDP

Application

Physical

Data Link

IP

Physical

Data Link

IP

TCP | UDP

Application

Physical

Data Link

IP


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IP header:


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IP header fields:

Version:Should have value 4 for IPv4Newer version of IP is IPv6

IHL:

Internet Header Length

Defines the header length in 32-bit words

Minimum length is 5; (5 x 4 = 20 bytes)

Type of Service: (TOS)

Defines the level of quality of service that this packet demands

e.g. in terms of delay, bandwidth, reliability or cost

Total Length:

Total length of complete packet in bytes

Maximum packet size is 64K bytes


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IP header fields: cont

Identification:All fragments of the same message have sameidentification value

Helps to reassemble fragments of the same message

Flags:DF = dont fragmentMF = more fragments

Fragment Offset:

Measured in multiple of 8-bytesContains offset of fragment from the beginning of theoriginal message

Helps to in-order reassembly of fragments


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Fragmentation example:


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Time To Live: (TTL)Is initialized by originator of packetDecremented by one at each intermediate router

If value of TTL is not zero then node process the packetotherwise discard the packet

Helps to limit the lifetime of packets

Protocol:Defines the upper layer protocol that will handle thepacket

e.g. TCP=6, UDP=17

Header Checksum:Is used to detect any error in header during transmission


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Source Address:IP address of packet originating node

Destination Address:IP address of the ultimate destination of the packet

This address is used by the routers to make the routingdecision

Options:Optional field

e.g. source routing, timestamp, etcPadding:

Ensures that the packet is aligned to 32-bit boundary

Only used when optional field is used


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IP Address:

IPv4 are 32-bit addressesRepresented as four 8-bit integer valuesCalled dotted-decimal notation

172.16.0.153

IPv6 are 128-bit addresses

Represented as eight 16-bit hexadecimal values separated by colons

5CFA:0002:0000:0000:CF07:1234:5678:FFCD

IP addresses are logical address values

Depends upon topological location of network

Independent of underlying hardware technology/addressesi.e. MAC addresses

Our current discussion is related to the IPv4 addresses


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IP address of my system:


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IPv4 Address classes:

IP address is divided in subparts:IP address = Network Part + Host PartNumber of bits assigned to network part or host part isnot fix

All nodes in a single network share commonnetwork part, but have different host part

Routers take forwarding decision only on the basisof network part

How many bits are assigned to network part iscritical

Historically it is determined by the class of IPaddress


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Assigning IP addresses:

NW = 192.68.5NW = 130.10

NW = 70

70.0.0.100

70.160.0.10170.0.5.102

130.10.2.100

130.10.4.102 130.10.2.101

192.68.5.10

192.68.5.11192.68.5.12


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IP Address classes:


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IP Address classes: cont

Class is identified from the left most byte value

Class A:From 0000 0000 to 0111 1111

i.e. from 0 to 127 e.g. 10.78.13.100

7-bits nw part = 128 different networks worldwide

24-bits host part = 16M hosts in each network

Class B:From 1000 0000 to 1011 1111

i.e. from 128 to 191 e.g. 172.16.0.153

14-bits nw part = 16K different networks worldwide

16-bits host part = 64K hosts in each network

Class C:From 1100 0000 to 1101 1111

i.e. from 192 to 223 e.g. 192.168.130.18

21-bits nw part = 2M different networks worldwide

8-bits host part = 256 hosts in each network


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IP Address classes: cont

Class D:

From 1110 0000 to 1110 1111

i.e. from 224 to 239 e.g. 230.100.0.99

No distinction of network/host part

Each address represents a multicast group address

28-bits = 256M number of different groups; enough!

Class E:

From 1111 0000 to 1111 0111

i.e. from 240 to onward

Has never been usedIdea was to reserve this range for future use

e.g. for experimentation purpose


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Special IP Addresses:

Broadcast Address:

All 32-bits are one

11111111.11111111.11111111.11111111

255.255.255.255

Loopback Address:

From 127.0.0.1 to 127.255.255.254Used for loopback testing even when system is not connected with network

Private Address Space:

Doesnt need to purchase these addresses

Reserved for local use. Routers dont forward packets with these addresses

Hence are not globally uniqueFrom 10.0.0.0 to 10.255.255.255 (Class-A)

From 172.16.0.0 to 172.31.255.255 (Class-B)

From 192.168.0.0 to 192.168.255.255 (Class-C)


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Global Uniqueness of IP addresses:

To communicate over global Internet, node shouldhave globally unique address

IANA is the global authority that is responsible forassigning unique internet identifiers such as:

Domain Names

IPv4/ IPv6 addresses

TCP/UDP port numbers

Internet Assigned Numbers Authority (IANA)Like IETF, IANA is also a part of InternetArchitecture Board (IAB)


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IANAs regional bodies:

There are three regional organizations under IANA:APNIC

Asia Pacific Network Information Center (APNIC)

Manages Asia & Australia

ARIN

American Registry for Internet Numbers (ARIN)

Manages North America, South America, Caribbean &Sub-Saharan Africa

RIPE

Rseaux IP Europens (RIPE)

Manages Europe & North Africa


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Obtaining Global IP address:

Regional Organizations like APNIC doesnt provideIP addresses directly to individual users

Internet Service Providers (ISPs) can purchase IPaddresses only in blocks

ISPs then provide these globally unique IPaddresses to its subscribers

ISP allocates these addresses either statically or

dynamically


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Obtaining Global IP address:

Static IP Address:Assigned permanently

Servers normally need addresses that are not changedover time

Cost is higher

Dynamic IP Address:Assigned temporarily

Can have new address in each session

Clients normally doesnt require permanent address

Services like DHCP, can manage dynamic addressassignment

Relatively of low cost


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NAT / NAPT:

Globally Unique IP addresses are precious

Organizations cant afford to purchase hundreds ofIP addresses for their LAN users

They often purchase small number of global IP

addresses, and operate their all LAN users withthem

Think! As a LAN user while connecting overInternet, you never use global addresses

There are two schemes:Network Address Translation (NAT)

Network Address & Port Translation (NAPT)


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NAT / NAPT Box:

Internet

NAT Box

Local networkwith many hosts


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Network Address Translation:

Purchase a pool of global IP addresses

Assign local private addresses inside the LAN

LAN users will use local addresses for localcommunication

When they wish to communicate globally, nodeswill attach their local address in the IP header

This packet will pass through the NAT box

NAT box will replace the local address with global

address without telling the local userThis is called translation from local address toglobal address


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NAT: cont

NAT box will maintain a table for all thesetranslations

When replies will arrive from the Internet,

NAT box will again translate the addresses,

but now from global address to the local address


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Example of NAT Box:

Internet

SA=192.168.0.11

(local)

SA=192.168.0.12(local)

SA=192.168.0.13(local)

SA=202.113.29.124(global)


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Static vs. Dynamic NAT

29Source: http://computer.howstuffworks.com/nat1.htm (05-11-2010)


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NAPT:

Network Address and Port Translation (NAPT)Also called Overloading NAT

And also Port Address Translation (PAT)

Rather than purchasing a pool of addresses, lets

put up with only single IP addressSame global address will be used for all LAN users

Uniqueness is maintained via port numbers

Very economical scheme


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NAPT:

31Source: http://computer.howstuffworks.com/nat1.htm (05-11-2010)


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Translation through NAPT:

32

Source: http://www.exfo.com/en/Library/WaveReview/WRArticle.aspx?Id=204 (08-11-2010)


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To be continued

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