Download Guide to TCP/IP, Third Edition

Guide to TCP/IP, Third
Edition
Chapter 2:
IP Addressing and Related Topics
Objectives
• Understand IP addressing, anatomy and
structures, and addresses from a computer’s point
of view
• Recognize and describe the various IP address
classes from A to E, and explain how they’re
composed and used
• Understand the nature of IP address limitations,
and how techniques like Classless Inter-Domain
Routing and Network Address Translation ease
those limitations
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Objectives (continued)
• Define the terms subnet and supernet, and apply
your knowledge of how subnets and supernets
work to solve specific network design problems
• Understand how public and private Internet
addresses are assigned, how to obtain them, and
how to use them properly
• Recognize the importance and value of an IP
addressing scheme
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IP Addressing Basics
• Computers deal with network addresses as bit
patterns
• IP uses a three-part addressing scheme
– Symbolic
• Example “support.dell.com”
– Logical numeric
• Example 172.16.1.10
– Physical numeric
• Six-byte numeric address, burned into firmware (on a
chip) by network interface manufacturers
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IP Addressing Basics (continued)
• Address Resolution Protocol (ARP)
– Permits computers to translate numeric IP
addresses to MAC layer addresses
• ReverseARP (RARP)
– Translates MAC layer addresses into numeric IP
addresses
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Anatomy of an IP Address
• IP addresses
– Dotted decimal notation
– Take the form n.n.n.n, where n is guaranteed to be
between 0 and 255
– Each number is an 8-bit number called an octet
– Duplication is not allowed
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IP Address Classes
• IP addresses
– Subdivided into five classes: Class A to Class E
• For first three classes octets are divided as follows
– Class A n.
h.h.h
– Class B n.n.
h.h
– Class C n.n.n.
h
• n = network, h = host
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IP Address Classes (continued)
• Address Classes D and E are for special uses
– Class D addresses
• Multicast communications
– Class E addresses
• Reserved entirely for experimental use
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More About Class A Addresses
• Class A addresses in binary form
– 0bbbbbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb
– b can be 1s or 0s
• Reserved for special uses
– Addresses consisting of all 0s and all 1s
• Reserved for private network use
– Address for network 10 (00001010)
• Reserved for loopback testing
– Address 127.n.n.n
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More About Class B Addresses
• Class B addresses take the following binary form
– 10bbbbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb
– b can be 1s or 0s
• 214 – 2
– Maximum number of usable network addresses
• 16,366
– Maximum number of public IP addresses
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More About Class C Addresses
• Class C addresses take the following binary form
– 110bbbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb
– b can be 1s or 0s
• 221 – 2
– The maximum number of usable network addresses
• Reserved for private use
– 256 Class C addresses, from 192.168.0.0 to
192.168.255.255
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More About Address Classes D and E
• Class D addresses
– 1110bbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb
– b can be 1s or 0s
– Multicast addresses
• Class E addresses
– 11110bbb.bbbbbbbb.bbbbbbbb.bbbbbbbb
– b can be 1s or 0s
– Only for experimental purposes
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Network, Broadcast, Multicast, and
Other Special IP Addresses
• Network address
– Any IP address where all host bits are “0”
• Broadcast address
– Address that all hosts on a network must read
• Broadcast traffic
– Seldom forwarded from one physical network to
another
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Broadcast Packet Structures
• IP broadcast packets have two destination address
fields
– Data Link layer destination address field
– Destination network address field
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Multicast Packet and Address
Structures
• IP gateway
– Router or other device that will forward traffic to the
host’s physical network
• The Internet Corporation for Assigned Names and
Numbers (ICANN)
– Allocates multicast addresses on a controlled basis
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The Vanishing IP Address Space
• Address space saving techniques
– Classless Inter-Domain Routing (CIDR)
– Trade in existing IP network addresses
– RFC 1918
• Reserves three ranges of IP addresses for private use
– Network Address Translation (NAT)
• Lets networks use private IP addresses internally and
maps them to public IP address externally
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Understanding Basic Binary Arithmetic
• Four binary calculations must be mastered
– Converting binary to decimal
– Converting decimal to binary
– Understanding how setting increasing numbers of
high-order bits to 1 in eight-bit binary numbers
corresponds to specific decimal numbers
– Understanding how setting increasing low-order bits
to 1 in eight-bit binary numbers corresponds to
specific decimal numbers
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Converting Decimal to Binary
• Converting decimal number 125 to binary
125 divided by 2 equals 62, remainder 1
62 divided by 2 equals 31, remainder 0
31 divided by 2 equals 15, remainder 1
15 divided by 2 equals 7, remainder 1
7 divided by 2 equals 3, remainder 1
3 divided by 2 equals 1, remainder 1
1 divided by 2 equals 0, remainder 1
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Converting Binary to Decimal
• Count the total number of digits in the number
• Subtract 1 from the total (8 - 1 = 7)
• Convert to exponential notation, using all the digits
as multipliers
• 11011011converts as follows
– 11011011 =
1*27+1*26+0*25+1*24+1*23+0*22+1*21+1*20 =
128+64+0+16+8+0+2+1 = 219
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High-Order Bit Patterns
Binary
10000000
11000000
11100000
11110000
11111000
11111100
11111110
11111111
Decimal
128
192
224
240
248
252
254
255
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Low-Order Bit Patterns
Binary
00000001
00000011
00000111
00001111
00011111
00111111
01111111
11111111
Decimal
1
3
7
15
31
63
127
255
IP Addressing and Related Topics
Exponent
21 - 1
22 - 1
23 - 1
24 - 1
25 - 1
26 - 1
27 - 1
28 - 1
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IP Networks, Subnets, And Masks
• Subnet mask
– Special bit pattern that “blocks off ” the network
portion of an IP address with an all-ones pattern
• Default masks for Classes A, B, and C
Class
Class A
Class B
Class C
Layout
n
h.h.h
n.n
h.h
n.n.n
h
IP Addressing and Related Topics
Default Mask
255.0.0.0
255.255.0.0
255.255.255.0
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IP Subnets and Supernets
• Subnetting
– Stealing (borrowing) bits from the host portion to
further subdivide the network portion of an address
• Supernetting
– Stealing bits from network portion
• Using them to create a single, larger contiguous
address space for host addresses
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Calculating Subnet Masks
• Types of subnet masking techniques
– Constant-length subnet masking (CLSM)
– Variable-length subnet masking (VLSM)
• In a VLSM addressing scheme
– Different subnets may have different extended
network prefixes
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Designing a Constant-Length Subnet
Mask
• Decide how many subnets are needed
• Add 2 to number of subnets needed then jump to
the nearest higher power of two
• Reserve bits of host portion’s address from the top
down
• Be sure that there are enough host addresses left
over on each subnet to be usable
• If using RIP
– Use the formula 2b – 2 to calculate the number of
usable subnets from a mask
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Designing a Variable-Length Subnet
Mask
• Analyze requirements for individual subnets
• Aggregate requirements by their relationships to
the nearest power of two
• Use subnets that require largest number of devices
– To decide the minimum size of the subnet mask
• Aggregate subnets that require fewer of hosts
• Define VLSM scheme that
– Provides the necessary number of subnets of each
size to fit its intended use best
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Calculating Supernets
• Supernets
– “Steal” bits from network portion of an IP address to
“lend” those bits to the host
– Permit multiple IP network addresses to be
combined
– Allow an entire group of hosts to be reached through
a single router address
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Classless Inter-Domain Routing
• Limitations
– Network addresses must be contiguous
– When address aggregation occurs
• CIDR address blocks work best when they come in
sets that are greater than 1 and equal to some lowerorder bit pattern that corresponds to all 1s
– Addresses commonly applied to Class C addresses
– To use a CIDR address on any network
• Routers in routing domain must “understand” CIDR
notation
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Public Versus Private IP Addresses
• Private IP addresses ranges
– May be in the form of IP network addresses
• Address masquerading
– May be performed by boundary devices that include
proxy server capabilities
• Private IP address limitation
– Some IP services require a secure end-to-end
connection
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Public Versus Private IP Addresses
(continued)
• Public IP addresses
– Remain important for identifying all servers or
services that must be accessible to the Internet
• Most organizations need public IP addresses only
for two classes of equipment
– Devices that permit organizations to attach networks
to the Internet
– Servers designed to be accessible to the Internet
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Managing Access To IP Address
Information
• Reverse proxying
– Permits the proxy server to front for servers inside
the boundary
• Important service that proxy server provides
– Manages what source addresses appear in
outbound packets that pass through it
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Obtaining Public IP Addresses
• Public IP addresses
– Issued by ISPs
• IP renumbering
– Switching addresses on every machine that uses
address from old ISP to unique address obtained
from new ISP
• ICANN
– Manages all IP-related addresses, protocol
numbers, and well-known port addresses
– Assigns MAC layer addresses for use in network
interfaces
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IP Addressing Schemes
• IP addressing scheme constraints
–
–
–
–
–
Number of physical locations
Number of network devices at each location
Amount of broadcast traffic at each location
Availability of IP addresses
Delay caused by routing from one network to
another
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The Network Space
• Application Specific Integrated Circuits (ASICs)
– Hardware used by switches to make decisions
• Layer-3 switch
– Implements the layer-3 logic from the software into
its own ASICs
– Allows you to partition a large network into many
smaller subnets with almost no loss of performance
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The Host Space
• Reasons for using binary boundaries
– You may want to implement layer-3 switching to
reduce the broadcast traffic
– One day you will want to classify your traffic to apply
Quality of Service (QoS) or policies of some sort
– Can be applied to firewall rules
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Summary
• IP addresses
– Provide foundation for identifying individual network
interfaces on TCP/IP networks
• IP addresses
– Come in five classes named through E
• Understanding binary arithmetic
– Essential to knowing how to deal with IP addresses
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Summary (continued)
• Classless Inter-Domain Routing (CIDR)
– Permits network-host boundary to fall away from
octet boundaries
• Subnetting
– Permits additional bits to be taken from the host
portion of a network
• Address masquerading and address substitution
– Techniques used to hide internal network IP
addresses from outside view
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Summary (continued)
• Within the Class A, B, and C IP address ranges
– IETF has reserved private IP addresses or address
ranges
• Internet Corporation For Assigned Names and
Numbers (ICANN)
– Ultimate authority for obtaining public IP addresses
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