Chapter 1: A First Look at Windows 2000 Professional
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Transcript Chapter 1: A First Look at Windows 2000 Professional
Managing IP
Addresses
and Broadcasts
Chapter 2
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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
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allocated addresses are wasted
Fear that the Internet may run out of
usable IP addresses
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Wasting
Addresses
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.
Consider this alternative addressing scheme:
192.168.0.192/30
192.168.0.0/25
This network allows 126
different host addresses
This network allows just
2 host addresses
192.168.0.128/26
This network allows 62
different 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
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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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Example of Routing Table with
Multiple Routes to a Destination
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Without Route Summarization
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With Route Summarization
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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
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Example of Calculating the Network Number
INSERT FIGURE 2-4
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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
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Example of Logical AND Operation
INSERT TABLE 2-1
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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
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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
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Example: Calculating Subnets with VLSM
192.168.10.64/27
192.168.10.132 /30
A class C address of 192.168.10.0/24
has been allocated.
60 hosts
192.168.10.0/26
12 hosts
12 hosts
192.168.10.96/28
Requirement levels, listed from the largest to the smallest:
Network
Perth LAN
KL LAN
Sydney
Singapore
Perth to KL
Sydney to KL
Singapore
to KL
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28 hosts
192.168.10.112 /28
2
4th Octet network/host
Host address range
.NNHHHHHHbits
/26 ( 62 hosts) 192.168.10.1 - 192.168.10.62
.NNNHHHHH /27 ( 30 hosts) 192.168.10.65 - 192.168.10.94
.NNNNHHHH /28 ( 14 hosts) 192.168.10.97 - 192.168.10.110
.NNNNHHHH /28 ( 14 hosts) 192.168.10.113 - 192.168.10.126
192.168.10.129 - 192.168.10.130
.NNNNNNHH /30 (2 hosts)
2
.NNNNNNHH /30 (2 hosts)
2
.NNNNNNHH /30 (2 hosts)
Hosts
60
28
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12
192.168.10.133 - 192.168.10.134
192.168.10.137 - 192.168.10.138
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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
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Cisco IOS Error Message for
Subnet with Discontiguous Bits
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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
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Route Summarization
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Network Numbers with VLSM
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Example: Route Aggregation with
VLSM
200.199.62.0 /25
200.199.62.128/25
200.199.63.0 /25
200.199.63.128/25
200.199.48.0/24
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
Advertise one supernet route:
200.199.62.0/23 to RTZ
_______________
Advertise one supernet route:
200.199.48.0/22 to RTZ
_______________
Advertise one supernet route:
200.199.32.0/19 to ISP
_______________
Advertise one supernet route:
200.199.32.0/20 to RTZ
_______________
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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
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Classful Routing
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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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Classful Address Distinctions
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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
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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)
RIRs assign addresses based on
Classless Inter-Domain Routing (CIDR)
CIDR
discussed in RFCs 1518, 1519, and
2050
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Each CIDR block has a prefix or IP
address and a prefix length or subnet
mask
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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
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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
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Discontiguous Subnets
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Outage Created by Discontiguous
Subnets
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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
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Example of IP Address Allocation
Based on Topology
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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
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Other Addressing Strategies
Unnumbered interfaces
Private address space
Network address translation
IP version 6
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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
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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
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through 172.31.255.255
Do not route addresses in these blocks to
the Internet
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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
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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
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Adoption of IPv6 is moving slowly
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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
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Creates administrative nightmare
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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
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Default UDP Ports
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