CSS 432: Subnetting, CIDR, and Global Internet1 CSS432 Subnetting and CIDR Textbook Ch3.2.5 Global...
Transcript of CSS 432: Subnetting, CIDR, and Global Internet1 CSS432 Subnetting and CIDR Textbook Ch3.2.5 Global...
CSS 432: Subnetting, CIDR, and Global Internet
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CSS432 Subnetting and CIDRTextbook Ch3.2.5
Global InternetTextbook Ch4.1
Professor: Munehiro Fukuda
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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…
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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
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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
200 nodesSubnet: 128.97.3.0
256 nodesSubnet: 128.97.4.0
Internet
IAS128.97.3.1-200
BBUS128.97.2.1-30
EDU128.97.1.1-30
CSS128.97.4.1 – 128.97.5.2
30 nodesSubnet: 128.97.1.0
Class B: 128.97.0.0
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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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
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! 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
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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
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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)