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CSS432 Subnetting and CIDR Textbook Ch3.2.5 Global Internet Textbook Ch4.1

CSS432 Subnetting and CIDR Textbook Ch3.2.5 Global Internet Textbook Ch4.1. Professor: Munehiro Fukuda. NSFNET backbone. Stanford. ISU. BARRNET. MidNet. …. regional. regional. Westnet. regional. Berkeley. P. ARC. UNL. KU. UNM. NCAR. UA. Internet Structure.

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CSS432 Subnetting and CIDR Textbook Ch3.2.5 Global Internet Textbook Ch4.1

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  1. CSS432 Subnetting and CIDRTextbook Ch3.2.5Global InternetTextbook Ch4.1 Professor: Munehiro Fukuda CSS 432: Subnetting, CIDR, and Global Internet

  2. NSFNET backbone Stanford ISU BARRNET MidNet … regional regional Westnet regional Berkeley P ARC UNL KU UNM NCAR UA 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? CSS 432: Subnetting, CIDR, and Global Internet

  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 CSS 432: Subnetting, CIDR, and Global Internet

  4. Internet Internet 256 nodes Subnet: 128.97.4.0 200 nodes Subnet: 128.97.3.0 40 nodes: Class C 200 nodes: Class C 256 nodes: Class B 40 nodes Subnet: 128.97.2.0 30 nodes Subnet: 128.97.1.0 30 nodes: Class C EDU 128.97.1.1-30 EDU 128.96.34.1 - 30 BBUS 128.97.2.1-30 BBUS 128.96.35.1-40 IAS 128.96.36.1-200 IAS 128.97.3.1-200 CSS 128.97.0.1 – 128.97.1.2 CSS 128.97.4.1 – 128.97.5.2 Subnetting - Concept Flat networks A collection of subnets • 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. Class B: 128.97.0.0 CSS 432: Subnetting, CIDR, and Global Internet

  5. Network number Host number Class B address 111111111111111111111111 00000000 Subnet mask (255.255.255.0) Network number Subnet ID Host ID Subnetted address Subnetting – How to Address • Subnet masks define variable partition of host part • Subnets visible only within site 127.97.0.1 – 127.97.255.254 # of bits in subnet mask 127.97.8.254/24 Subnet ID CSS 432: Subnetting, CIDR, and Global Internet

  6. Subnetting – How to Address Note: subnet all 0’s and all 1’s are not recommended CSS 432: Subnetting, CIDR, and Global Internet

  7. Subnet mask: 255.255.255.128 Subnet number: 128.96.34.0/25 128.96.34.15 128.96.34.1 H1 R1 Subnet mask: 255.255.255.128 128.96.34.130 Subnet number: 128.96.34.128/25 128.96.34.139 128.96.34.129 H2 R2 H3 128.96.33.1 128.96.33.14 Subnet mask: 255.255.255.0 Subnet number: 128.96.33.0/24 Subnet Example IP address & subnet mast = subnet number Example: 128.96.34.15 & 255.255.255.128 = 128.96.34.0 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 CSS 432: Subnetting, CIDR, and Global Internet

  8. Forwarding Algorithm D = destination IP address for 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 CSS 432: Subnetting, CIDR, and Global 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 CSS 432: Subnetting, CIDR, and Global Internet

  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 CSS 432: Subnetting, CIDR, and Global Internet

  11. Corporation X 11000000 00000100 0000 192.4.0.0/20 Internet backbone Regional network Corporation Y 11000000 00000100 0001 192.4.16.0/20 Corporation Z 11000000 00000100 0011 192.4.48.0/20 Border gateway 11000000 00000100 00 192.4.0.0/18 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 CSS 432: Subnetting, CIDR, and Global Internet

  12. 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0 0 0 Classless Lookup • Binary Tree for IP Routing • Patricia Tree for IP Routing 128.4.0.0/18 10000000 00000100 00 65.4.0.0/14 01000001 000001 192.4.48.0/20 11000000 00000100 0011 192.4.0.0/18 11000000 00000100 00 192.4.48.0/20 11000000 00000100 0011 192.4.0.0/18 11000000 00000100 00 Skip 10 Skip 4 128.4.0.0/18 10000000 00000100 00 65.4.0.0/14 01000001 000001 CSS 432: Subnetting, CIDR, and Global Internet

  13. NSFNET backbone Stanford ISU BARRNET MidNet … regional regional Westnet regional Berkeley P ARC UNL KU UNM NCAR UA 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) AS2 AS1 R2 R1 Interior Exterior CSS 432: Subnetting, CIDR, and Global Internet

  14. Popular Interior Gateway Protocols • RIP: Route Information Protocol • Distributed with Unix • Distance-vector algorithm • Based on hop-count • OSPF: Open Shortest Path First • Recent Internet standard • Uses link-state algorithm • Supports load balancing • Supports authentication CSS 432: Subnetting, CIDR, and Global Internet

  15. Large corporation “ ” Consumer ISP Peering point Backbone service provider Peering point Consumer ” ISP “ “ Consumer ISP ” Large corporation Small corporation 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 • 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. CSS 432: Subnetting, CIDR, and Global Internet

  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 CSS 432: Subnetting, CIDR, and Global Internet

  17. 128.96 Customer P 192.4.153 (AS 4) Regional provider A (AS 2) Customer Q 192.4.32 (AS 5) 192.4.3 Backbone network (AS 1) Customer R 192.12.69 (AS 6) Regional provider B (AS 3) Customer S 192.4.54 (AS 7) 192.4.23 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 CSS 432: Subnetting, CIDR, and Global Internet

  18. BGP Messages BGP Speaker (receiver) BGP Speaker (sender) OPEN( myAS#, timeRequiredToReceiveTheRestOfMessage, myIP, options) KEEPALIVE( ) UPDATE( #Withdrawn, #MaskBits, IP, …., #NewPath, #MaskBits, IP, …., PathAttributes ) TCP connection KEEPALIVE( ) Next UPDATE( ) or KEEPALIVE( ) Next UPDATE( ) or KEEPALIVE( ) Timer Interrupt NOTIFICATION(ErrorCode, ErroSubCode, Data) Timeline CSS 432: Subnetting, CIDR, and Global Internet

  19. BGP Configuration for Cisco Routers AS 777 (friend) AS 102 R_f R_A AS 101 (myself) R_me AS 103 R_B NO ! Our AS number is 101, provide A’s is 102, provide B’s 103, and our firend’s is 777 router 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 routes ip 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 routes ip as-path access-list 82 deny ^102_ ! Don’t let provider A use my network for transit ip as-path access-list 82 permit .* ! ! Define an AS path access-list that blocks provider A’s routes ip as-path access-list 83 deny ^103_ ! Don’t let provider B use my network for transit Ip as-path access-list 83 permit .* at the end at the beginning CSS 432: Subnetting, CIDR, and Global Internet

  20. Area 3 Area 1 Area 0 R7 R9 R3 R1 R8 R4 R2 Area 2 Virtual Link NO R5 R6 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. CSS 432: Subnetting, CIDR, and Global Internet

  21. Router 2 Router 1 Router 3 172.168.48.0/24 172.168.50.0/24 Router 4 Router 5 172.168.49.0/24 172.168.0.0/24 OSPF Configuration for Cisco Routers ! Router 1 starts an OSPF process and place all interfaces in area 0 router 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 1 router 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 0 router 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 CSS 432: Subnetting, CIDR, and Global Internet

  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 CSS 432: Subnetting, CIDR, and Global Internet

  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) CSS 432: Subnetting, CIDR, and Global Internet

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