Download 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

4
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

10
to telephone area code
Reduces number of routes to a specific
customer
10
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
12
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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