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

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

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

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

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 Host ID Subnet ID 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

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 25 255.255.255.128 26 255.255.255.192 27 255.255.255.224 28 255.255.255.240 29 255.255.255.248 255.255.255.252 31 255.255.255.254 32 255.255.255.255 Note: subnet all 0’s and all 1’s are not recommended CSS 432: Subnetting, CIDR, and Global Internet

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

CSS 432: Subnetting, CIDR, and Global Internet 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

CSS 432: Subnetting, CIDR, and Global Internet 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

CSS 432: Subnetting, CIDR, and Global Internet 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

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

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

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

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

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 Peering point Large corporation Small corporation “ 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. CSS 432: Subnetting, CIDR, and Global Internet

CSS 432: Subnetting, CIDR, and Global Internet 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

CSS 432: Subnetting, CIDR, and Global Internet 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) 128.96 192.4.153 192.4.32 192.4.3 192.12.69 192.4.54 192.4.23 CSS 432: Subnetting, CIDR, and Global Internet

CSS 432: Subnetting, CIDR, and Global Internet BGP Messages BGP Speaker (sender) BGP Speaker (receiver) 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

CSS 432: Subnetting, CIDR, and Global Internet BGP Configuration for Cisco Routers AS 102 AS 777 (friend) R_A R_f AS 101 (myself) R_me R_B AS 103 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

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

CSS 432: Subnetting, CIDR, and Global Internet OSPF Configuration for Cisco Routers Router 1 Router 2 Router 3 Router 4 Router 5 172.168.48.0/24 172.168.50.0/24 172.168.0.0/24 172.168.49.0/24 ! 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 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 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

CSS 432: Subnetting, CIDR, and Global Internet 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 other options CSS 432: Subnetting, CIDR, and Global Internet

CSS 432: Subnetting, CIDR, and Global Internet 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