gofortution

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IP Addressing and IP Addressing and Introduction to IP routingIntroduction to IP routing

Avgust Jauk <[email protected]>

ARNES

Bratislava, August 98

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AgendaAgenda

Internet topology Introduction to addressing Idea of routing Special address conventions Classfull addressing Classless addressing Routing protocols: IGPs and EGPs

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Internet topology Internet topology

Internet - Network of Networks Networks

– Based on different technology– Large or small– Fast or slow– Variety of connected nodes

Routers (Gateways) Protocols

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Internet protocol stackInternet protocol stack

OSI Reference ModelTCP/IP

Conceptual Layers

7 Application

6 Presentation Application ftp, telnet, smtp,

5 Session snmp, tftp, etc.

4 Transport Transport TCP, UDP

3 Network InternetIP, ICMP, ARP,RARP

2 Data Link Network Ethernet,

1 Physical Interface FDDI, etc.

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Internet protocol Internet protocol dependenciesdependencies

Hardware

Users

Application ProgramsNFS

FTP SNMP CMOT XDR

SMTP rlogin& rsh

TELNET DNS ASN.1 TFTP BOOTP RPC

TCP UDPIP (plus ICMP)

ARP RARPHARDWARE LINK LEVEL AND ACCESS PROTOCOLS

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0 4 8 16 19 24 31

VERS HLEN SERVICE TYPE TOTAL LENGTH

IDENTIFICATION FLAGS FRAGMENT OFFSET

TIME TO LIVE PROTOCOL HEADER CHECKSUM

SOURCE IP ADDRESS

DESTINATION IP ADDRESS

IP OPTIONS (IF ANY) PADDING

DATA

. . .

Internet datagram formatInternet datagram format

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ICMP datagram formatICMP datagram format

0 4 8 16 19 24 31

VERS HLEN SERVICE TYPE TOTAL LENGTH

IDENTIFICATION FLAGS FRAGMENT OFFSET

TIME TO LIVE PROTOCOL=1 HEADER CHECKSUM

SOURCE IP ADDRESS

DESTINATION IP ADDRESS

IP OPTIONS (IF ANY) PADDING

ICMP type ICMP code ICMP checksum

ICMP message-specific data

ICMP message-specific data

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ICMP Message typesICMP Message types

Type Field ICMP Message Type

0 Echo Reply 3 Destination Unreachable 4 Source Quench 5 Redirect (change a route) 8 Echo Request 11 Time Exceeded for Datagram 12 Parametere Problem on a Datagram 13 Timestamp Request 14 Timestamp Reply 15 Information Request (obsolete) 16 Information Reply (obsolete) 17 Address Mask Request 18 Address Mask Reply

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Introduction to addressingIntroduction to addressing

Do I need an address? What types of addresses are there?

– Postal address– Telephone number– In Computer Networks:

Physical Addresses (Ethernet, FDDI, ...) Textual Addresses - Names Network level addresses (IP, X.25,...)

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Addressing in the InternetAddressing in the Internet

Address specifies host’s interface 32 bit addresses Network part & Host part

Dotted decimal notation: 192.164.2.4

Network part Host part

0 31

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Idea of routingIdea of routing

Routers forward datagrams between connected networks

They need to know via which interface to send a datagram

Routing decisions are based on the information stored in the routing table

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Routing tableRouting table

Tells where to send datagram for a particular network

Network Next-Hop Port Metric

194.181.200.0 194.181.208.1 Eth0 1193.2.1.0 194.181.208.320 Eth1 14153.5.0.0 194.181.214.25 Fddi0 80.0.0.0 194.181.210.1 S0 5

Next-Hop routers must be directly reachable

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Routing table (cont.)Routing table (cont.)

Default Route - a special entry in the routing table:– “Pass all datagrams for unknown networks to

this router”– Represented by the entry for network 0.0.0.0

Routing uses network part of the address!

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Routing AlgorithmRouting Algorithm

Extract destination IP address from datagram Extract network address from the IP address If destination network equals my network

– Send directly to destination using physical network Else If destination address matches a host-

specific route in the routing table:– Send to the router specified in the routing table

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Routing Algorithm (cont.)Routing Algorithm (cont.)

Else if destionation network matches a network in the routing table– Send to the router specified in the routing entry

Else If there is a default route in the routing table:– Send to the router specified in the default route entry

Else:– Send a “No route to host” message to the source

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Populating the Routing TablePopulating the Routing Table

Manually by network administrator: Static Routes– No dynamic changes to these routes will accur

Dynamically by routing protocol– Routing info is exchanged between routers– The routing “metric” is used to find the best

path

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Static RoutesStatic Routes

Manually configured by network administrator

A

B

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Static RoutesStatic Routes

Router cannot automatically reroute if path fails

A

B

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Routing protocolsRouting protocols

Routers use a common protocol to exchange routing information

Best path between networks or subnets is determined by “Routing Metric”

Automatic adaption to topology changes

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Routing protocolsRouting protocols

64 kbps

64 kbps2 Mbps

2 Mbps

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Special address conventionsSpecial address conventions

Broadcast Addresses– Directed broadcast: host part all 1’s -

194.181.200.255– Limited broadcast: all 1’s - 255.255.255.255

0 means “This”– host part = 0 - this host– network part = 0 - this network– miss used as a broadcast address

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Special address conventions Special address conventions (cont.)(cont.)

Loopback Address : 127.0.0.1 – for testing and inter-process communication on

the local machine– should never appear on any network

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Summary of special address Summary of special address conventionsconventions

all 0s

all 0s host

all 1snet

127 anything (often 1)

all 1s

This host

Limited broadcast (local net)

Host on this net

Directed broadcast for net

Loopback

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Classess and address formatsClassess and address formats

0 1 2 3 4 8 16 24 31

netid

netid

hostid

hostid

multicast address

reserved for future use

Class A

Class C

Class D

Class E 0

0

0

0

netid hostidClass B 01

1 1

1 1 1

1 1 1 1

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Classes: How to recognize Classes: How to recognize themthem

Class A: first byte in range 1-126 Class B: first byte in range 128-191 Class C: first byte in range 192-223 Class D: first byte in range 224-239 Class E: first byte in range 240-255

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Classes: Size and NumberClasses: Size and Number

Class A: 16.777.214 hosts, 128 networks Class B: 65.534 hosts, 16.324 networks Class C: 254 hosts, 2.097.152 networks

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Problems with ClassesProblems with Classes

Class A usually to big Class C often to small Not enough Class Bs Inefficient utilisation of address space Solution: extending the network part of the

address: Subnetting

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SubnettingSubnetting

Class B 01

Class B 01

Class B Address: Before Subnetting

Class B Address: After Subnetting

Network

Network Host

Host

Subnet

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Subnet maskSubnet mask

Subnet mask defines the network part– binary 1 in network bits– binary 0 in hosts bits

Subnet mask must be contiguous!

Network part Host part0 31

1 ..... 1 0 ...... 0

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Subnetting (cont.)Subnetting (cont.)

Not limited to byte border Subnets “0” and “-1” used to be reserved

– Subnet “0” : this subnet– Subnet “-1”: broadcast

Network administrator decides on the subnet size

Network and subnet numbers used for routing decisions

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Subnetting and routing Subnetting and routing

one subnet mask per particular class routing considerations

– all subnets of the same class must be contiguous

– or static routes must be used– or routing protocol must carry also subnet

masks

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Subnetting and routingSubnetting and routing

all subnets of the same class must be contiguous!

C11C12

B

C14C13

C1C1

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Subnet mask bitsSubnet mask bits128 64 32 16 8 4 2 1

1 0 0 0 0 0 0 0 = 128

1 1 0 0 0 0 0 0 = 192

1 1 1 0 0 0 0 0 = 224

1 1 1 1 0 0 0 0 = 240

1 1 1 1 1 0 0 0 = 248

1 1 1 1 1 1 0 0 = 252

1 1 1 1 1 1 1 0 = 254

1 1 1 1 1 1 1 1 = 255

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Binary NumbersBinary Numbers

= 128 + 64 + 32 + 2

128 64 32 16 8 4 2 1

Represent 226 decimal in binary:

=

27 62 25 24 23 22 12

1 1 1 0 0 0 1 0

022

2762 25 24 23 22 12 022

226=

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Subnetting a Class CSubnetting a Class C

split subnet mask # subnets # hosts/subnet total # hosts utilis.

1:7 128 2 252126 99%2:6 192 4 24862 98%3:5 224 8 24030 94%4:4 240 16 14 224 88%5:3 248 32 6 192 76%6:2 252 64 2 128 50%7:1 254 / / / /

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Variable Length Subnet Variable Length Subnet Masks (VLSM)Masks (VLSM)

Subnets are of different size A means for conserving address space How to do it:

– how big is the biggest subnet?– split the class into such pieces – split (“sub-subnet” ) those peieces further

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VLSM (cont.)VLSM (cont.)

How to do VLSM0 255

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VLSM and routingVLSM and routing

Prerequisites:– routing protocol must carry subnet masks– or static routes must be used

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Classfull Addressing: Classfull Addressing: drawbacksdrawbacks

Classfull Addressing + Subnetting– at least one route per class is advertised in routing

updates Number of networks is doubling faster than once

per year Memory is not growing that fast Only a few routers can keep the current number of

routes Route flapping

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Classless addressingClassless addressing

Introduced by CIDR - Classless InterDomain Routing

Networks are grouped (aggregated) into blocks Blocks of networks are advertised New way of thinking:

– there are no networks numbers, but just address space prefixes

– there are no subnet masks, just prefix lenghts

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Classless addresses notationClassless addresses notation

10.181.215.32 /27 10.181.215.32 with mask

255.255.255.224 binary representation of mask:

11111111.11111111.11111111.11100000

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Classless address notationClassless address notation

Hosts

. . .8163264128256. . .40968192163843276865535. . .

Prefix

. . ./29/28/27/26/25/24. . ./20/19/18/17/16. . .

Classful

. . .

1 C. . .16 C’s32 C’s64 C’s128 C’s1 B. . .

Subnet Mask

. . .255.255.255.248255.255.255.240255.255.255.224255.255.255.192255.255.255.128255.255.255.0. . .255.255.240.0255.255.224.0255.255.192.0255.255.128.0255.255.0.0. . .

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Classless network Classless network aggregation - Supernettingaggregation - Supernetting

Class C 24-bit prefix 11000000

192 168 64 0

10101000 01000000

Class C next24-bit prefix 11000000

192 168 65 0

10101000 01000001 00000000

Prefix Host part

00000000

Commonprefix: 23 bits 11111111 11111111 1111111 0 00000000

00000000

Classless23-bit prefix 11000000

192 168 64 /23

10101000 0100000 0 00000000

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Classless network Classless network aggregation (cont.)aggregation (cont.)

Before aggregation– 201.222.191.0/24– 201.222.192.0/24– 201.222.193.0/24

After aggregation– 201.222.191.0/24– 201.222.192.0/23

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Classless addressing and Classless addressing and routingrouting

Longest match routing Route distr. between two protocols, one is

not supporting classless– use a default route– “explode” supernet info. into individual

network numbers

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Classes of routing protocolsClasses of routing protocols

The early Arpanet was completelly flat - single “network” model– one routing protocol, all routers had all the routing info– with the growth it become hard to maintaine and

computationally intensive Solution: split the Internet into a set of

Autonomous Systems (AS)– Each Autonomous System is a set of routers and

networks under the same administration

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Classes of routing protocols Classes of routing protocols (cont.)(cont.)

Special routers, called “Exterior gateways” used to connect ASes

Two classes of routing protocols:– Interior routing protocols (IGP - Interior

Gateway protocols)– Exterior routing protocols (EGP - Exterior

Gateway protocols)

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Interior Routing Protocols Interior Routing Protocols (IGPs)(IGPs)

Used inside an Autonomous System Designed to handle more redundant links

– Links are cheaper in a local environment => one can afford more redundant links

Designed with a higher bandwidth in mind– Cheaper bandwidth => one can use more

bandwidth for the exchange of routing information

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Interior Routing Protocols Interior Routing Protocols (cont.)(cont.)

They generally contaion less ingformation than EGPs– IGPs in general (with exeptions) do not have to know

about any other network outside the AS No policy support

– Inside AS, one generally does not want to aplly policy– everyone can use every available link– policies are generally only set on what links should be

preffered

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Interior Routing Protocols Interior Routing Protocols (cont.)(cont.)

Fairly extensive metric support – Redudancy => one has to distinguish between

redundant links– metrics or “costs” help in the decision proccess

Designed for fast convergence– Because of the redudancy, IGPs are designed to

make quick changes if the network topology changes

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Exterior Routing Protocols Exterior Routing Protocols (EGPs)(EGPs)

Used to exchange routing information between ASes

Designed with lower bandwidth in mind– long distance links are more expensive => routing

protocol should use less bandwidth for the exchange of routing information

They generally contain a lot of information– EGPs have to know about all external networks

– In the Internet that might be 40.000 networks

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Exterior Routing Protocols Exterior Routing Protocols (cont.)(cont.)

They assume a less reliable network– most of them are connection oriented for reliable

delivery They are designed to provide policy control

– generally you set routing policy at the border of your routing domain

They do not run in every single router– Only at the border of your AS you have to run an EGP– Internal routers can be less powerfull

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SummarySummary

We have covered– Internet topology– Routing:

static, dynamic classes of routing protocols

– Addressing classfull

– subnetting– VLSM

classless

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Where to get more informationWhere to get more information

RFC´s (RFC-1880: Internet Official Protocol Standards)

Books– D.C.Lynch, M.T.Rose: Internet System

Handbook– D.E.Comer: Internetworking with TCP/IP

Mailing lists Usenet News