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Managing IP Addresses and Broadcasts

Managing IP Addresses and Broadcasts. Chapter 2. Making Networks Scalable. A scalable network grows continually, yet smoothly and stably Avoid problems with growing networks by providing redundancy and designing networks for easy manageability

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Managing IP Addresses and Broadcasts

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  1. Managing IP Addresses and Broadcasts Chapter 2

  2. Making Networks Scalable • A scalable network grows continually, yet smoothly and stably • Avoid problems with growing networks by providing redundancy and designing networks for easy manageability • Choice of routing protocol greatly influences scalability of network

  3. The Growth of the Internet • Initially, Internet was small and limited to researchers • In 1990s, Internet grew immensely as governments, universities, corporations, and the general public began to use it • Organizations and Internet now experiencing problems managing IP addresses

  4. IP Address Exhaustion • 32-bit IP addresses provide, in theory, over four billion addresses • Many allocated addresses are wasted • Fear that the Internet may run out of usable IP addresses

  5. Wasting Addresses Consider this alternative addressing scheme: 192.168.0.192/30 192.168.0.128/26 192.168.0.0/25 Consider the following example: In this network a Class C address with a 255.255.255.0 mask has been used for each subnet 192.168.2.0/24 192.168.1.0/24 192.168.3.0/24 The WAN link has enough IP addresses for 254 separate hosts, but will use only two. Each LAN has enough IP addresses for 254 separate hosts. Broadcasts would be a major issue if this address space were not further subnetted. This network allows 62 different host addresses This network allows 126 different host addresses This network allows just 2 host addresses It is acceptable to use subnet zero and the all-ones subnet with VLSM. (In the past, use of the first and last subnets was discouraged).

  6. Routing Table Growth • Internet routing table increased from about 5000 routers in 1990 to more than 100,000 in 2001 • Large routing tables require more CPU time and more memory • Result in slowed down table lookups • Make troubleshooting more difficult

  7. Managing IP Addresses • Administrators use many strategies to manage IP addresses • Hierarchical addressing • Hierarchical routing • Route summarization • Variable-length subnet masks • Classful and classless routing

  8. Hierarchical Addressing • Layered, orderly addressing • Similar to public telephone network • Local office recognizes local exchange • Local central office forwards long distance calls to central office in other area codes • Calls then treated as local call by central office in other area codes

  9. Hierarchical Routing • Router forwards packet to core layer router based on first octet IP address • Core layer router forwards packet to distribution layer router based on first two octets • Distribution layer router forwards packet to access layer router based on first three octets • Access layer router forwards packet to final destination

  10. Route Summarization • Also called address aggregation • Combines multiple routes that share leftmost bits into one summary route • Similar to telephone area code • Reduces number of routes to a specific customer

  11. Route Summarization INSERT FIGURE 2-2

  12. Route Summarization • If router has both summary route and ordinary route, it selects the one with the longest match • Looks at length of prefix or number of bits in subnet mask to determine path • Route summarization does not make address allocation more efficient, especially point-to-point links

  13. Example of Routing Table with Multiple Routes to a Destination

  14. Without Route Summarization

  15. With Route Summarization

  16. Variable-Length Subnet Masks • VLSMs, defined in RFC 1812, let you subdivide Class C • Subnet mask helps router break IP address into network and host portions • Router uses network part of IP address to forward packet to correct network • Local router uses host part of IP address to deliver packet to destination

  17. Example of Calculating the Network Number INSERT FIGURE 2-4

  18. The Logical AND Operation • Router matches bits in IP address and subnet mask • Compares bits and performs logical AND operation • If both bits are ones, the result is a one • If either bit is a zero, the result is a zero • Logical AND operation provides network number

  19. Example of Logical AND Operation INSERT TABLE 2-1

  20. Calculating Subnets • Number of subnets depends on number of bits borrowed from network portion of IP address • Calculate number of new subnets by 2n, where n is the number of borrowed bits • Subtract two to find number of usable host bits • First and last addresses reserved for network address and broadcast address

  21. Classful and Classless Netmasks • If netmask follows traditional class boundaries, it is called classful routing • If netmask does not follow traditional class boundaries, it is called classless routing • Can supernet or use a smaller netmask than traditional class boundaries

  22. Requirement levels, listed from the largest to the smallest: Network Hosts 4th Octet network/host bits Host address range Perth LAN 60 KL LAN 28 Sydney 12 Singapore 12 Perth to KL Sydney to KL Singapore to KL Example: Calculating Subnets with VLSM 192.168.10.64/27 28 hosts A class C address of 192.168.10.0/24 has been allocated. 192.168.10.136 /30 192.168.10.128 /30 192.168.10.132 /30 60 hosts 12 hosts 12 hosts 192.168.10.0/26 192.168.10.96/28 192.168.10.112 /28 192.168.10.1 - 192.168.10.62 .NNHHHHHH /26 ( 62 hosts) .NNNHHHHH /27 ( 30 hosts) 192.168.10.65 - 192.168.10.94 .NNNNHHHH /28 ( 14 hosts) 192.168.10.97 - 192.168.10.110 192.168.10.113 - 192.168.10.126 .NNNNHHHH /28 ( 14 hosts) 192.168.10.129 - 192.168.10.130 2 .NNNNNNHH /30 (2 hosts) 2 .NNNNNNHH /30 (2 hosts) 192.168.10.133 - 192.168.10.134 192.168.10.137 - 192.168.10.138 2 .NNNNNNHH /30 (2 hosts)

  23. Calculating VLSM Subnet Masks • According to RFC 1812, all bits in subnet mask must be contiguous • Cisco IOS displays error message if subnet has discontiguous bits • Be sure routing protocol supports VLSMs • OSPF and EIGP support VLSMs • RIP version 1 and IGRP do not support VLSMs

  24. Cisco IOS Error Message for Subnet with Discontiguous Bits

  25. Summarizing Routes Using VLSMs • VLSMs allocate IP addresses more efficiently • VLSMs provide more flexibility in summarizing routes • Based entirely on higher-order bits they share on the left • Routes do not have to be contiguous • Prefix of summary route based on bits shared by all routes

  26. Route Summarization

  27. Network Numbers with VLSM

  28. 200.199.62.0 /25 200.199.62.128/25 200.199.63.0 /25 200.199.48.0/24 200.199.63.128/25 200.199.49.0/24 200.199.50.0/24 200.199.51.0/24 200.199.32.0/22 200.199.36.0/22 200.199.40.0/22 200.199.44.0/22 Example: Route Aggregation with VLSM Advertise one supernet route: _______________ to RTZ 200.199.62.0/23 Advertise one supernet route: _______________ to RTZ 200.199.48.0/22 Advertise one supernet route: _______________ to ISP 200.199.32.0/19 Advertise one supernet route: _______________ to RTZ 200.199.32.0/20

  29. Classes of IP Addresses • Class depends on first octet of IP address • Class A addresses begin with a zero as the leftmost bit; use 8 bits for network address • Class B addresses begin with a 10 as the first two bits; use 16 bits for network address • Class C addresses begin with a 110 as the first three bits; use 24 bits for network address • Class D addresses are used for multicast • Class E addresses are used for research

  30. Classful Routing • Router uses classes of addresses • Can subnet along class octet boundaries • Routing protocols include RIPv1 and IGRP • May use IP classless global configuration command to forward packets to a summary route • Classful routing is inflexible, limited, and sometimes wasteful

  31. Classful Address Distinctions

  32. Classless Routing • Ignores traditional class boundaries • Protocols include OSPF and EIGRP • Can allocate and receive IP addresses as necessary • Previously Three Regional Internet Registries (RIRs) now Five, allocate IP classless addresses in blocks • American Registry for Internet Numbers (ARIN) • Réseaux IP Européens Network Coordination Centre (RIPE NCC) • Asia Pacific Network Information Center (APNIC) • Regional Latin-America and Caribean Address Registry (LACNIC)-2002 • African Network Information Centre (AfriNIC)-2005

  33. Classless Inter-Domain Routing (CIDR) • RIRs assign addresses based on Classless Inter-Domain Routing (CIDR) • CIDR discussed in RFCs 1518, 1519, and 2050 • Each CIDR block has a prefix or IP address and a prefix length or subnet mask

  34. Allocating IP Addresses • How IP addresses are allocated affects how well network performs • Pitfalls of route summarization • Requires more planning • More useful with classless routing protocol • Can lead to poor path selection • Can create problem with discontiguous subnets

  35. Problems with Summarization and Discontiguous Subnets • Route summarization hides details of network from routers • Discontiguous subnets may result in outage or inability to deliver packets

  36. Discontiguous Subnets

  37. Outage Created by Discontiguous Subnets

  38. Allocating IP Addresses Using VLSMs • Efficient allocation of IP addresses requires • Allocating enough IP addresses to each subnet for future growth • Not allocating more than necessary for each subnet • Plan for route summarization • Do not assign IP addresses haphazardly • Assign IP addresses based on topology of network

  39. Example of IP Address Allocation Based on Topology

  40. Process of Assigning IP Addresses • After finding baseline subnet, calculate the number of subnets you can use • Cisco recommends allocating addresses from the lowest to the highest for easier summarizing of routes • Put your largest networks into the lower subnets

  41. Other Addressing Strategies • Unnumbered interfaces • Private address space • Network address translation • IP version 6

  42. Unnumbered Interfaces • Configure IP on interface without explicitly using an IP address • Use ip unnumbered command to refer to an existing interface that routers use as source address • Unnumbered interfaces often get IP address from loopback address • Drawbacks include inability to get status by pinging, making troubleshooting and monitoring more difficult • Some serial protocols such as X.25 and SMDS do not support unnumbered interfaces

  43. Private Address Space • RCF 1918 sets aside three ranges of IP addresses for private networks • 10.0.0.0/8 • 192.168.0.0/16 • 172.16.0.0 through 172.31.255.255 • Do not route addresses in these blocks to the Internet

  44. Network Address Translation • NAT involves device such as a router that translates one set of IP addresses into another set • Can conserve IP addresses by translating a large pool of private addresses into a small pool of public addresses • Disadvantages include increased latency and difficulties with protocols or applications that put IP address in data portion of IP packet

  45. IP Version 6 • IPv6, specified in RFC 2460, offers several advantages over current version (IPv4) • Uses 128 bit IP addresses • Provide over 3 x 1038 possible IP addresses • Includes more support for quality of service and better security • Adoption of IPv6 is moving slowly

  46. Managing Broadcasts • Routers do not, by default, forward broadcasts • If PC boots without knowing its IP address, it must contact DHCP or BOOTP server • If server not on same segment, PC cannot get an IP address • Can hard code all IP addresses if PC unable to reach server • Creates administrative nightmare

  47. Using a Helper Address • Solution is to allow broadcasts in specific situations • Cisco routers can direct a broadcast to a helper address • Can configure more than one helper address • Must use IP directed-broadcast interface configuration command with Cisco IOS 12.0 and later • Configure helper address to router closest to client • By default, helper address command turns on eight UDP ports as shown in Table 2-8

  48. Default UDP Ports

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