css 432: subnetting, cidr, and global internet1 css432 subnetting and cidr textbook ch3.2.5 global...

CSS 432: Subnetting, CIDR, and Global Internet

1

CSS432 Subnetting and CIDRTextbook Ch3.2.5

Global InternetTextbook Ch4.1

Professor: Munehiro Fukuda


2

Internet Structure

Autonomous System (AS): Administered independently of other AS Have a different routing protocol and metrics

Do we really need to give an independent class A/B/C network number to every single AS?

NSFNET backboneStanford

BARRNET

regional

Berkeley

PARC

NCAR

UA

UNM

Westnet

regional

UNL KU

ISU

MidNet

regional…


3

Scaling Issues in Routing Inefficient use of IP Address Space

Class C with 2 hosts (2/255 = 0.78% efficient) Class B with 256 hosts (256/65535 = 0.39% efficient)

IP address space gets consumed too quickly

Too Many Networks Routing tables do not scale Route propagation protocols do not scale Router gets slower to scan a big forwarding table

Hierarchy


4

Subnetting - Concept

Problem: Internet identifies only classes Four networks must receive an independent class of network number, (which

exhausts IP addresses and floods network #s.) Subnet: collects networks belonging to the same AS and give a single class of

network number, which is then divided into subnet numbers internally.

Flat networks A collection of subnets

40 nodes: Class C

200 nodes: Class C

256 nodes: Class B

Internet

IAS128.96.36.1-200

BBUS128.96.35.1-40

EDU128.96.34.1 - 30

CSS128.97.0.1 – 128.97.1.2

30 nodes: Class C

40 nodesSubnet: 128.97.2.0



Internet

IAS128.97.3.1-200

BBUS128.97.2.1-30

EDU128.97.1.1-30

CSS128.97.4.1 – 128.97.5.2


Class B: 128.97.0.0


5

Subnetting – How to Address Subnet masks define variable partition of host part Subnets visible only within site

Network number Host number

Class B address

Subnet mask (255.255.255.0)

Subnetted address

111111111111111111111111 00000000

Network number Host IDSubnet ID

127.97.0.1 – 127.97.255.254

127.97.8.254/24

Subnet ID

# of bits in subnet mask


6

Subnetting – How to Address#bits Subnetwork Mask #subnets in Class B #subnets in Class C # of hosts

16 255.255.0.0 1 - 65534

17 255.255.128.0 - - 32766

18 255.255.192.0 2 - 16382

19 255.255.224.0 6 - 8190

20 255.255.240.0 14 - 4094

21 255.255.248.0 30 - 2046

22 255.255.252.0 62 - 1022

23 255.255.254.0 126 - 510

24 255.255.255.0 254 1 254

25 255.255.255.128 510 0 126

26 255.255.255.192 1022 2 62

27 255.255.255.224 2046 6 30

28 255.255.255.240 4094 14 14

29 255.255.255.248 8190 30 6

30 255.255.255.252 16382 62 2

31 255.255.255.254 32766 126 -

32 255.255.255.255 65534 254 -

Note: subnet all 0’s and all 1’s are not recommended


7

Subnet Example

Forwarding table at router R1

Subnet Number Subnet Mask Next Hop

128.96.34.0 255.255.255.128 interface 0

128.96.34.128 255.255.255.128 interface 1

128.96.33.0 255.255.255.0 R2

Subnet mask: 255.255.255.128Subnet number: 128.96.34.0/25

128.96.34.15 128.96.34.1

H1R1

128.96.34.130Subnet mask: 255.255.255.128Subnet number: 128.96.34.128/25

128.96.34.129128.96.34.139

R2H2

128.96.33.1128.96.33.14

Subnet mask: 255.255.255.0Subnet number: 128.96.33.0/24

H3

IP address & subnet mast = subnet numberExample: 128.96.34.15 & 255.255.255.128= 128.96.34.0


8

Forwarding Algorithm

D = destination IP addressfor each entry (SubnetNum, SubnetMask, NextHop) D1 = SubnetMask & D if D1 = SubnetNum if NextHop is an interface deliver datagram directly to destination else deliver datagram to NextHop (a router)

Use a default router if nothing matches Not necessary for all 1s in subnet mask to be contiguous

But should be avoided Can put multiple subnets on one physical network

Ex. Two or more departments want to have their own subnet and to allocate IP addresses in it while sharing just one physical network

Subnets not visible from the rest of the Internet


9

Supernetting Subnetting

Purpose: divide a large class of network number into sub network numbers → help assign address carefully.

Problem: an AS with more than 255 hosts still needs class B.

Supernetting Solution: assign block of contiguous network numbers to an

institution. Ex. Assign two class C network numbers instead of one class B

network. Side effect: The information routers store and exchange

increases dramatically Ex. If an AS has 16 class C network numbers, every Internet

router needs 16 entries for this AS.

CIDR: Classless Inter-Domain Routing


10

CIDR Basic concept of supernetting using class C:

Represent blocks with a single pair (first_class_C_network_address, count)

Example (192.5.48.0, 3) Points to a group of blocks such as 192.5.48.0, 192.5.49.0, and 192.5.50.0

In practice No restriction to class C nor use of count Restrict block sizes to powers of 2 Use a bit mask (CIDR mask) to identify block size

Ex. An AS assigned a block of 2048 contiguous addresses starting at 128.211.168.0, (i.e., a collection of 8 class C networks)

Lowest 128.211.168.0 10000000 11010011 10101000 00000000

Highest 128.211.175.255 10000000 11010011 10101111 11111111

CIDR mask (21bits) 11111111 11111111 11111000 00000000 Address Notation: 128.211.168.0/21


11

Classless Addressing Examples CIDR allows to aggregate routes repeatedly

Then, what if there is a router capable of forwarding packets both to the regional network and to the cooperation Z? Prefix Next Hop 192.4.0.0/18 the regional network 192.4.48.0/20 corporation Z To which of those two should we forward a packet destined to 192.4.48.3?

Use Principle of Longest Match

Regional networkCorporation Y

11000000 00000100 0001192.4.16.0/20

Corporation X11000000 00000100 0000

192.4.0.0/20

Internetbackbone

Border gateway11000000 00000100 00192.4.0.0/18

Corporation Z11000000 00000100 0011

192.4.48.0/20


12

Classless Lookup Binary Tree for IP Routing

Patricia Tree for IP Routing

192.4.0.0/18 11000000 00000100 00

192.4.48.0/20 11000000 00000100 0011

128.4.0.0/18 10000000 00000100 00

65.4.0.0/14 01000001 000001

0

1

1

0

1

00

0 00

00 1

1

01

0

1

0

0Skip 10

1 11

Skip 4

65.4.0.0/14 01000001 000001128.4.0.0/18 10000000 00000100 00

192.4.0.0/18 11000000 00000100 00

192.4.48.0/20 11000000 00000100 0011


13

Route Propagation Know a smarter router

Hosts know local (default) routers Local routers know site routers Site routers know core router Core routers know everything Site routers are called border routers.

Autonomous System (AS) Corresponds to an administrative domain Examples: University, company, backbone network

Two-level route propagation hierarchy Interior gateway protocol (each AS selects its own) Exterior gateway protocol (Internet-wide standard)

NSFNET backboneStanford

BARRNETregional

BerkeleyPARC

NCAR

UA

UNM

Westnetregional

UNL KU

ISU

MidNetregional…

AS1 AS2

R1 R2

Exterior

Interior


14

Popular Interior Gateway Protocols RIP: Route Information Protocol

Distributed with UnixDistance-vector algorithmBased on hop-count

OSPF: Open Shortest Path FirstRecent Internet standardUses link-state algorithmSupports load balancing Supports authentication


15

Well-known Exterior Gateway Protocol

Border Gateway Protocol – 4th Version (BGP-4) Assumption: Internet as an arbitrarily interconnected set

of ASs Goal: Reachability than optimality

Backbone service provider

Peeringpoint

Peeringpoint

Large corporation

Large corporation

Smallcorporation

“Consumer ” ISP

“Consumer” ISP

“ Consumer” ISP

Stub AS: A single connecitoin to another AS Only carries local traffic.

Multihomed AS: Connections to multi ASs Refuses to carry transit traffic

Transit AS: Connections to multi-ASs Carries both transit and local traffic.


16

BGP-4 Each AS has:

one or more border routers one BGP speaker that advertises:

local networks other reachable networks (transit AS only) gives complete path information

Characteristics Inter-BGP speaker communication based on P2P and TCP Consistent maintenance on routing information among multiple BGP

speakers Reachability-based information Policy Support to distinguish between intra- and inter-AS

reachability information Incremental updates that sends only reachability change Route aggregation to send multiple routes in one message Authentication to allow a receiver to authenticate messages


17

BGP Example Speaker for AS2 advertises reachability to P and Q

network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS2

Speaker for backbone advertises networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be

reached along the path (AS1, AS2). Speaker can cancel previously advertised paths

Backbone network(AS 1)

Regional provider A(AS 2)

Regional provider B(AS 3)

Customer P(AS 4)

Customer Q(AS 5)

Customer R(AS 6)

Customer S(AS 7)

128.96192.4.153

192.4.32192.4.3

192.12.69

192.4.54192.4.23


18

BGP MessagesBGP Speaker (sender) BGP Speaker (receiver)

Timeline

OPEN( myAS#, timeRequiredToReceiveTheRestOfMessage, myIP, options)

KEEPALIVE( )

UPDATE( #Withdrawn, #MaskBits, IP, …., #NewPath, #MaskBits, IP, …., PathAttributes )

KEEPALIVE( )

Next UPDATE( ) or KEEPALIVE( )

Next UPDATE( ) or KEEPALIVE( )

NOTIFICATION(ErrorCode, ErroSubCode, Data)

Timer Interrupt

TCP connection


19

AS 102

AS 103

AS 101(myself)

! Our AS number is 101, provide A’s is 102, provide B’s 103, and our firend’s is 777router bgp 101 network 172.16.0.0 neighbor 172.16.1.1 remote-as 102 ! Provider A neighbor 172.16.1.1 filter-list 81 weight 100 ! Traffic to our friend 777 uses provider A neighbor 172.16.1.1 filter-list 82 out neighbor 172.16.2.7 remote-as 103 ! Provider B neighbor 172.16.2.7 filter-list 83 out!! Define an AS path access-list that selects our friend’s routesip as-path access-list 81 permit _777$ ! There is a path through A to our friend 777!! Define an AS path access-list that blocks provider B’s routesip as-path access-list 82 deny ^102_ ! Don’t let provider A use my network for transitip as-path access-list 82 permit .*!! Define an AS path access-list that blocks provider A’s routesip as-path access-list 83 deny ^103_ ! Don’t let provider B use my network for transitIp as-path access-list 83 permit .*

BGP Configuration for Cisco Routers

R_me

R_B

R_AAS 777(friend)

R_f

NO

at the end

at the beginning


20

Routing Areas AS divided into areas Area 0

Known as the backbone area and connected to the back bone

Routers (R1, R2, R3) called ABR (Area Border Router)

OSPF link states do not leave the area in

which they originated if they are not ABRs.

ABRs summarize routing information that they have learned from one area and make it available in their advertisements to other areas.

R4

R5R6

R2

R3R1

R7

R8

R9

Area 0

Area 3

Area 2

Area 1

NO

Virtual Link


21

! Router 1 starts an OSPF process and place all interfaces in area 0router ospf 1 network 0.0.0.0 255.255.255.255 area 0 ! Mask here means wild cards 0 or 1.

! Router 2 starts an OSPF process and place all interfaces in area 1router ospf 1 network 0.0.0.0 255.255.255.255 area 0

! Router 3 starts an OSPF process and place in area1 the interface that is part ! of 172.168.48.0./22. Place all the other interfaces in area 0router ospf 1 network 172.168.48.0 0.0.3.255 area 1 ! The last 10 bits may be 0 or 1 netwrok 0.0.0.0 255.255.255.255 area 0

OSPF Configuration for Cisco Routers

Router 1 Router 2

Router 3

Router 4 Router 5

172.168.48.0/24

172.168.50.0/24

172.168.0.0/24172.168.49.0/24


22

IP Version 6 Features

128-bit addresses (classless) multicast real-time service authentication and security autoconfiguration end-to-end fragmentation protocol extensions

Header 40-byte “base” header extension headers (fixed order, mostly fixed length)

fragmentation source routing authentication and security other options


23

Reviews Subnetting: How to address and forwarding algorithm Supernetting: CIDR, principle of longest match, and

classless lookup Exterior gateway protocol: BGP and routing areas

Exercises in Chapter 3 Ex. 55 (Subnetting) Ex. 68 (CIDR) Ex. 72 (CIDR) Ex. 74 (CIDR)

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