Managing ip addresses and broadcasts
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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 l.jpg

Managing IP Addresses and Broadcasts

Chapter 2

Making networks scalable l.jpg
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

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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

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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

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Wasting Addresses

Consider this alternative addressing scheme:

Consider the following example:

In this network a Class C address with a mask has been used for each subnet

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).

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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

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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

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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

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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

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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

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Route Summarization


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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

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Variable-Length Subnet Masks Destination

  • 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

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Example of Calculating the Network Number Destination


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The Logical AND Operation Destination

  • 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

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Example of Logical AND Operation Destination


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Calculating Subnets Destination

  • 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

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Classful and Classless Netmasks Destination

  • 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

Example calculating subnets with vlsm l.jpg

Requirement levels, listed from the largest to the smallest: Destination



4th Octet network/host bits

Host address range

Perth LAN








Perth to KL

Sydney to KL

Singapore to KL

Example: Calculating Subnets with VLSM

28 hosts

A class C address of has been allocated. /30 /30 /30

60 hosts

12 hosts

12 hosts /28 -

.NNHHHHHH /26 ( 62 hosts)

.NNNHHHHH /27 ( 30 hosts) -

.NNNNHHHH /28 ( 14 hosts) - -

.NNNNHHHH /28 ( 14 hosts) -


.NNNNNNHH /30 (2 hosts)


.NNNNNNHH /30 (2 hosts) - -


.NNNNNNHH /30 (2 hosts)

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Calculating VLSM Subnet Masks Destination

  • 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

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Summarizing Routes Using VLSMs Destination

  • 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

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Route Summarization Destination

Example route aggregation with vlsm l.jpg /25 Destination /25

Example: Route Aggregation with VLSM

Advertise one supernet route: _______________ to RTZ

Advertise one supernet route: _______________ to RTZ

Advertise one supernet route: _______________ to ISP

Advertise one supernet route: _______________ to RTZ

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Classes of IP Addresses Destination

  • 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

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Classful Routing Destination

  • 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

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Classless Routing Destination

  • 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

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Classless Inter-Domain Routing (CIDR) Destination

  • 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

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Allocating IP Addresses Destination

  • 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

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Problems with Summarization and Discontiguous Subnets Destination

  • Route summarization hides details of network from routers

  • Discontiguous subnets may result in outage or inability to deliver packets

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Allocating IP Addresses Using VLSMs Destination

  • 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

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Process of Assigning IP Addresses Destination

  • 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

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Other Addressing Strategies Destination

  • Unnumbered interfaces

  • Private address space

  • Network address translation

  • IP version 6

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Unnumbered Interfaces Destination

  • 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

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Private Address Space Destination

  • RCF 1918 sets aside three ranges of IP addresses for private networks



    • through

  • Do not route addresses in these blocks to the Internet

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Network Address Translation Destination

  • 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

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IP Version 6 Destination

  • 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

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Managing Broadcasts Destination

  • 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

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Using a Helper Address Destination

  • 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

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Default UDP Ports Destination