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


The growth of the internet l.jpg
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


Ip address exhaustion l.jpg
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


Wasting addresses l.jpg
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).


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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


Hierarchical routing l.jpg
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


Route summarization l.jpg
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

INSERT FIGURE 2-2


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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

INSERT FIGURE 2-4


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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


Example of logical and operation l.jpg
Example of Logical AND Operation Destination

INSERT TABLE 2-1


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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

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)


Calculating vlsm subnet masks l.jpg
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



Summarizing routes using vlsms l.jpg
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


Route summarization26 l.jpg
Route Summarization Destination



Example route aggregation with vlsm l.jpg

200.199.62.0 /25 Destination

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


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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


Classful routing l.jpg
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



Classless routing l.jpg
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


Classless inter domain routing cidr l.jpg
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


Allocating ip addresses l.jpg
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


Problems with summarization and discontiguous subnets l.jpg
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




Allocating ip addresses using vlsms l.jpg
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



Process of assigning ip addresses l.jpg
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


Other addressing strategies l.jpg
Other Addressing Strategies Destination

  • Unnumbered interfaces

  • Private address space

  • Network address translation

  • IP version 6


Unnumbered interfaces l.jpg
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


Private address space l.jpg
Private Address Space Destination

  • 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


Network address translation l.jpg
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


Ip version 6 l.jpg
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


Managing broadcasts l.jpg
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


Using a helper address l.jpg
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


Default udp ports l.jpg
Default UDP Ports Destination


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