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IP Addressing and CIDR

IP Addressing and CIDR. IP Addresses . IP Addresses. What is an IP Address? . An IP address is a unique global address for a network interface Exceptions: Dynamically assigned IP addresses (  DHCP) IP addresses in private networks ( NAT) An IP address: - is a 32 bit long identifier

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IP Addressing and CIDR

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  1. IP Addressing and CIDR

  2. IP Addresses

  3. IP Addresses

  4. What is an IP Address? • An IP address is a unique global address for a network interface • Exceptions: • Dynamically assigned IP addresses ( DHCP) • IP addresses in private networks ( NAT) • An IP address: - is a 32 bit long identifier - encodes a network number (network prefix) and a host number

  5. Hosts, Networks, and Routers Host 7 Host 1 Network A Host 2 Host 1 Router Network C Network B Unique IP Address = Network Number + Host Number Host 12 Host 2

  6. 11111111 00010001 10000111 00000000 Network Number Host Number IP Addresses come in two parts Where is this dividing line? Well, that depends ....

  7. Network C, Host 3 Network A, Host 3 Network B, Host 77 Actually, IP addresses Identify Interfaces Host 7 Host 1 Network A Host 2 Host 1 Network C Network B Machines can have more than one IP address. All routers do! Host 12 Host 2

  8. IP Forwarding Table Destination Next Hop Interface Net A Router 1 INT 7 Net B Direct INT 4 Net C, Host 3 Router 2 INT 3 Net C Router 1 INT 7 A destination is usually a network. May also be a host, or a “gateway of last resort” (default) The next hop is either a directly connected network or a router on a directly connected network A physical interface

  9. IP Forwarding Process 1. Remove a packet from an input queue 2. Check for correctness decrement TTL field 4. Place packet on correct output queue Forwarding Process 3. Match packet’s destination to a table entry If queues get full, just drop packets! If queues get full, just drop packets! IP Forwarding Table Router

  10. IGP EGP EGP IGP IGP EGP Architecture of Routing Protocols Interior Gateway Protocols (IGP) : inside autonomous systems Exterior Gateway Protocols (EGP) : between autonomous systems AS 701 UUNet OSPF, IS-IS, RIP, EIGRP, ... BGP Policy Based Metric Based AT&T Common Backbone Sprint AS 6431 AS 7018

  11. The Most Common Routing Protocols BGP RIP Cisco proprietary UDP OSPF IS-IS TCP EIGRP IP (and ICMP) Routing protocols exchange network reachability information between routers.

  12. What is a Routing Process? Manual configuration import information from other routers export information to other routers Routing Process Protocol-Specific Routing Table OS kernel IP Forwarding Table Router

  13. OSPF Process OSPF Routing tables RIP Process RIP Routing tables BGP Process BGP Routing tables Many routing processes can run on a single router BGP OS kernel RIP Domain OSPF Domain IP Forwarding Table

  14. Basic Architectural Componentsof an IP Router Routing Protocols Routing Table Control Plane Datapath per-packet processing Forwarding Table Switching

  15. Two components of routing • Control component • Decides where the packets will go • Use a set of routing protocols (e.g. OSPF, BGP) to collect information and produce a “forwarding table” • “Control plane” • Forwarding component • Moving packets from input to output ports according to forwarding table and packet header • “Forwarding plane” Routing “daemon” collect routing info and maintain routing DB routes kernel Forwarding table Forwarding algorithm and mechanism packets

  16. 11111111 00010001 10000111 00000000 Network Number Host Number IP Addresses come in two parts Where is this dividing line? Well, that depends ....

  17. Classful Addresses hhhhhhhh 0nnnnnnn hhhhhhhh hhhhhhhh Class A 10nnnnnn nnnnnnnn hhhhhhhh hhhhhhhh Class B nnnnnnnn nnnnnnnn hhhhhhhh 110nnnnn Class C h = host identifier bit n = network address bit

  18. The Classful Address Space Leads to very inefficient allocation of addresses …

  19. Problems with Classful IP Addresses • By the early 1990s, the original classfull address scheme had a number of problems • Flat address space. Routing tables on the backbone Internet need to have an entry for each network address. By the 1993, the size of the routing tables started to outgrow the capacity of routers (C networks). Other problems: • Too few network addresses for large networks • Class A and Class B addresses were gone • Limited flexibility for network addresses: • Class A and B addresses are overkill (>64,000 addresses) • Class C address is insufficient (requires 40 Class C addresses)

  20. Allocation of Classful Addresses

  21. Flat Network Addressing Y P Exports at least 12 network addresses T W This router needs at least 12 table entries X R N Q U S V Z

  22. Adds Multiple Entries to Routing Tables Wastes IP Addresses 2000 Allocated 63,534 Wasted Class B Class C Network ID Network ID Host ID Host ID 255 255 255 255 255 0 0 0 w w x x y y z z Company Network IDs Internet 192.168.1.0 192.168.2.0 Network of 2000 Computers Assigned 65,534 IP Addresses 192.168.3.0 Portion of Internet Routing Tables 192.168.1.0 255.255.255.0 192.168.1.1 192.168.2.0 255.255.255.0 192.168.2.1 192.168.3.0 255.255.255.0 192.168.3.1 192.168.4.0 255.255.255.0 192.168.4.1 192.168.5.0 255.255.255.0 192.168.5.1 192.168.6.0 255.255.255.0 192.168.6.1 192.168.7.0 255.255.255.0 192.168.7.1 192.168.8.0 255.255.255.0 192.168.8.1 192.168.4.0 192.168.5.0 192.168.6.0 192.168.7.0 192.168.8.0 Limitations of Classful IP Addressing

  23. Subnets • The idea is to share the same IP network number among multiple subnets • Subnets of a network should reside in the same general locale (e.g., college campus, corporate location, …) • Routers on an IP network know their local subnets • Remote routers need to know only the network address

  24. Subnetting Subnetting • Problem: Organizations have multiple networks which are independently managed • Solution 1: Allocate a separate network address for each network • Difficult to manage • From the outside of the organization, each network must be addressable. • Solution 2: Add another level of hierarchy to the IP addressing structure University Network Engineering School Medical School Library

  25. Address assignment with subnetting • Each part of the organization is allocated a range of IP addresses (subnets or subnetworks) • Addresses in each subnet can be administered locally University Network 128.143.0.0/16 Engineering School Medical School 128.143.71.0/24128.143.136.0/24 128.143.56.0/24 Library 128.143.121.0/24

  26. Basic Idea of Subnetting • Split the host number portion of an IP address into a subnet number and a (smaller) host number. • Result is a 3-layer hierarchy • Then: • Subnets can be freely assigned within the organization • Internally, subnets are treated as separate networks • Subnet structure is not visible outside the organization network prefix host number network prefix subnet number host number extended network prefix

  27. Subnetmask • Routers and hosts use an extended network prefix (subnetmask) to identify the start of the host numbers

  28. Advantages of Subnetting • With subnetting, IP addresses use a 3-layer hierarchy: • Network • Subnet • Host • Reduces router complexity. Since external routers do not know about subnetting, the complexity of routing tables at external routers is reduced. • Note: Length of the subnet mask need not be identical at all subnetworks.

  29. Example: Subnetmask • 128.143.0.0/16 is the IP address of the network • 128.143.137.0/24 is the IP address of the subnet • 128.143.137.144 is the IP address of the host • 255.255.255.0 (or ffffff00) is the subnetmask of the host • Use of subnetting or length of the subnetmask is decided by the network administrator • Consistency of subnetmasks is the responsibility of administrator

  30. No Subnetting • All hosts think that the other hosts are on the same network

  31. With Subnetting • Hosts with same extended network prefix belong to the same network

  32. Variable Length Subnet Masks • With only a single subnet mask across a network prefix, an organization was locked into a fixed number of fixed size subnets • When a network is assigned more than one subnet mask, it is considered a network with “variable length subnet masks” since extended-network-prefixes have different lengths • VLSM permits slicing and dicing subnets into different sizes and therefore numbers of hosts in subnets under a single Network ID, thereby minimizing, if not eliminating, wasted addresses.

  33. With Subnetting • Different subnetmasks lead to different views of the size of the scope of the network

  34. 12.253.0.0/19 12.253.32.0/19 12.253.64.0/19 12.253.96.0/19 12.253.128.0/19 12.253.160.0/19 12.253.192.0/19 Mask may vary with location 12.0.0.0/16 : : : 12.1.0.0/16 12.3.0.0/24 12.2.0.0/16 12.3.1.0/24 : : 12.3.0.0/16 : : : 12.0.0.0/8 12.3.254.0/24 This allows all of these (sub)networks to be aggregated into one entry in an IP forwarding table 12.253.0.0/16 12.254.0.0/16

  35. Hierarchical Network Addressing(Subnetting and Supernetting) Network Z-T T Exports only one Address P M Q This is called Aggregation W Z This Router needs only 3 table entries for networks Z-T, Z-S, and Z-U S M P W F S K J X U Network Z-U-X Network Z

  36. After Subnetting Routing Table for Router B Routing Table for Router B Before Supernetting: Routing Table for Router B After Supernetting: 220.78.168.0 255.255.255.0 220.78.168.1 220.78.168.0 255.255.255.0 220.78.168.1 220.78.168.0 255.255.255.0 220.78.168.1 220.78.168.0 220.78.169.0 255.255.255.0 220.78.168.1 220.78.168.0 220.78.168.64 220.78.170.0 255.255.255.0 220.78.168.1 220.78.169.0 220.78.171.0 255.255.255.0 220.78.168.1 220.78.168.128 220.78.172.0 255.255.255.0 220.78.168.1 Router A 220.78.170.0 220.78.173.0 255.255.255.0 220.78.168.1 220.78.168.192 Router B Router A Router B Router B 220.78.174.0 255.255.255.0 220.78.168.1 220.78.171.0 220.78.175.0 255.255.255.0 220.78.168.1 220.78.169.0 220.78.172.0 RouterA 220.78.169.64 220.78.168.0 220.78.173.0 220.78.169.128 220.78.174.0 220.78.169.192 220.78.175.0 220.78.170.0 220.78.170.64 Optimizing the Allocation of IP Addresses

  37. CIDR - Classless Interdomain Routing • IP backbone routers have one routing table entry for each network address: • With subnetting, a backbone router only needs to know one entry for each Class A, B, or C networks • This is acceptable for Class A and Class B networks • 27 = 128 Class A networks • 214 = 16,384 Class B networks • But this is not acceptable for Class C networks • 221 = 2,097,152 Class C networks • In 1993, the size of the routing tables started to outgrow the capacity of routers • Consequence: The Class-based assignment of IP addresses had to be abandoned

  38. CIDR - Classless Interdomain Routing • Goals: • New interpretation of the IP address space • Restructure IP address assignments to increase efficiency • Permits route aggregation to minimize route table entries • CIDR (Classless Interdomain routing) • abandons the notion of classes • Key Concept:The length of the network prefix in the IP addresses is kept arbitrary • Consequence: Size of the network prefix must be provided with an IP address

  39. CIDR Notation • CIDR notation of an IP address: 192.0.2.0/18 • "18" is the prefix length. It states that the first 18 bits are the network prefix of the address (and 14 bits are available for specific host addresses) • CIDR notation can replace the use of subnetmasks (but is more general) • IP address 128.143.137.144 and subnetmask 255.255.255.0 becomes 128.143.137.144/24 • CIDR notation allows to drop trailing zeros of network addresses: 192.0.2.0/18 can be written as 192.0.2/18

  40. Why do people still talk about • CIDR eliminates the concept of class A, B, and C networks and replaces it with a network prefix • Existing classfull network addresses are converted to CIDR addresses: 128.143.0.0  128.143.0.0/16 • The change has not affected many (previously existing) enterprise networks • Many network administrators (especially on university campuses) have not noticed the change

  41. CIDR address blocks • CIDR notation can nicely express blocks of addresses • Blocks are used when allocating IP addresses for a company and for routing tables (route aggregation) CIDR Block Prefix # of Host Addresses /27 32 /26 64 /25 128 /24 256 /23 512 /22 1,024 /21 2,048 /20 4,096 /19 8,192 /18 16,384 /17 32,768 /16 65,536 /15 131,072 /14 262,144 /13 524,288

  42. CIDR and Address assignments • Backbone ISPs obtain large block of IP addresses space and then reallocate portions of their address blocks to their customers. Example: • Assume that an ISP owns the address block 206.0.64.0/18, which represents 16,384 (214) IP addresses • Suppose a client requires 800 host addresses • With classful addresses: need to assign a class B address (and waste ~64,700 addresses) or four individual Class Cs (and introducing 4 new routes into the global Internet routing tables) • With CIDR: Assign a /22 block, e.g., 206.0.68.0/22, and allocated a block of 1,024 (210) IP addresses.

  43. Delegate IANA Allocate RIR / NIR Allocate Network Assign User Where do IP addresses come from? IETF

  44. IPv4 Address Allocation IANA: Internet Assigned Numbers Authority ARIN: American Registry for Internet Numbers Source: iana.org Total Addresses: 4,295m. US Commercial 369m. US Government 201m. Reserved (IANA) 1,896m. ARIN (N. America) 268m. Asia/Pacific 151m. Europe 218m. International 1,191m.

  45. CIDR and Routing • Aggregation of routing table entries: • 128.143.0.0/16 and 128.142.0.0/16 are represented as 128.142.0.0/15 • Longest prefix match: Routing table lookup finds the routing entry that matches the longest prefix What is the outgoing interface for 128.143.137.0/24 ? Route aggregation can be exploited when IP address blocks are assigned in an hierarchical fashion Routing table

  46. CIDR and Routing Information Company X : 206.0.68.0/22 ISP X owns: 206.0.64.0/18 204.188.0.0/15 209.88.232.0/21 Internet Backbone ISP y : 209.88.237.0/24 Organization z1 : 209.88.237.192/26 Organization z2 : 209.88.237.0/26

  47. CIDR and Routing Information Backbone routers do not know anything about Company X, ISP Y, or Organizations z1, z2. Company X : 206.0.68.0/22 ISP X owns: ISP y sends everything which matches the prefix: 209.88.237.192/26 to Organizations z1209.88.237.0/26 to Organizations z2 ISP X does not know about Organizations z1, z2. 206.0.64.0/18 204.188.0.0/15 209.88.232.0/21 Internet Backbone ISP X sends everything which matches the prefix: 206.0.68.0/22 to Company X, 209.88.237.0/24 to ISP y ISP y : 209.88.237.0/24 Backbone sends everything which matches the prefixes 206.0.64.0/18, 204.188.0.0/15, 209.88.232.0/21 to ISP X. Organization z1 : 209.88.237.192/26 Organization z2 : 209.88.237.0/26

  48. Classless Addressing (and CIDR) • Eliminated class boundaries • Introduced the notion of a variable length prefix between 0 and 32 bits long • Prefixes represented by P/l: e.g., 122/8, 212.128/13, 34.43.32/22, 10.32.32.2/32 etc. • An l-bit prefix represents an aggregation of 232-l IP addresses

  49. IPv6 - IP Version 6 • IP Version 6 • Is the successor to the currently used IPv4 • Specification completed in 1994 • Makes improvements to IPv4 (no revolutionary changes) • One (not the only !) feature of IPv6 is a significant increase in of the IP address to 128 bits (16 bytes) • IPv6 will solve – for the foreseeable future – the problems with IP addressing • 1024 addresses per square inch on the surface of the Earth.

  50. IPv6 Header

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