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IP Addressing and Related Topics
A Guide to TCP/IP
Chapter 2
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Objectives
After reading this chapter and completing the
exercises you will be able to:
• 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
Chapter 2
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Objectives
After reading this chapter and completing the
exercises you will be able to:
• 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
• Symbolic names are easier to remember a string,
such as www.course.com, than a numeric address,
such as 199.95.728—computers are the opposite
• They deal with network addresses in the form of bit
patterns that translate into decimal numbers
• IP uses a three-part addressing scheme, as follows:
– Symbolic
– Logical numeric
– Physical numeric
Chapter 2
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IP Addressing Basics
• In keeping with the layered nature of network
models, it makes sense to associate the MAC layer
address with the Data Link layer (or TCP/IP Network
Access layer, if you prefer to think in terms of that
model), and to associate IP addresses with the
Network layer (or the TCP/IP Internet layer)
• As data moves through intermediate hosts between
the original sender and the ultimate receiver, it does
so between pairs of machines, where each pair
resides on the same physical network
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IP Addressing Basics
• At the Network layer, the original sender’s
address is represented in the IP source address
field in the IP packet header, and the ultimate
recipient’s address is represented in the IP
destination address field in the same IP packet
header
• The IP destination address value, in fact, is what
drives the sometimes-long series of intermediate
transfers, or hops, which occur as data makes its
way across a network from sender to receiver
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Anatomy of an IP Address
• Numeric IP addresses use dotted decimal
notation when expressed in decimal numbers,
and take the form n.n.n.n., in which n is
guaranteed to be between zero and 255 for each
and every value
• The numeric values in dotted decimal
representations of numeric IP addresses are
usually decimal values, but may occasionally
appear in hexadecimal (base 16) or binary (base
2) notation
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Anatomy of an IP Address
• Duplication of numeric IP addresses is not
allowed because that would lead to confusion
• Also, there is a notion of “neighborhood” when it
comes to interpreting numeric IP addresses
• Proximity between two numeric IP addresses
(especially if the difference is only in the
rightmost one or two octets) can sometimes
indicate that the machines to which those
addresses correspond reside close enough
together to be on the same general network, if not
on the same physical cable segment
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IP Address Classes
• Initially, these addresses were further subdivided into
five classes, from Class A to Class E
• For the first three classes of addresses, divide the octets
as follows to understand how they behave:
Class A
n
h.h.h
Class B
n.n
Class C
n.n.n
h.h
h
• If more than one octet is part of the network or host
portion of the address, then the bits are simply
concentrated to determine the numeric address
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IP Address Classes
• The network portion of that address is 10, whereas
the host portion is 12.120.2, treated as a three-octet
number
• Address Classes D and E are for special uses
• Class D addresses are used for multicast
communications, in which a single address may be
associated with more than one network host machine
• This is useful only when information is broadcast to
more than one recipient at a time so it should come
as no surprise that video and teleconferencing
applications, for example, use multicast addresses
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More About Class A Addresses
• Expressed in binary form (ones and zeroes only),
Class A addresses always take the form:
0bbbbbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb
• The leading digit is always zero, and all other
digits can be either ones or zeroes
• On any IP network, addresses consisting of all
zeroes and all ones are reserved for special uses,
so of those 128 possible network addresses, only
those from 00000001 to 01111110 (or 1 to 126, in
decimal terms) are considered usable
Chapter 2
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More About Class A Addresses
• The address for network 10 is reserved for private network
use
• Also, by convention, the address 127.n.n.n is reserved for
loopback testing (or checking the integrity and usability of
a TCP/IP protocol stack installed on any computer
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More About Class B Addresses
• Class B addresses always take the form:
10bbbbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb
• The leading two digits are 10, and the remaining
digits can be either ones or zeroes
• RFC 1918 stipulates that 16 Class B addresses,
from 172.16.0.0 to 172.32.255.255, are reserved
for private use
• This means that the maximum number of public
IP addresses for Class B is 16,382-16, or 16,366
Chapter 2
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Class B Address Facts and Figures
Chapter 2
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More About Class C Addresses
•
Class C addresses always take the form:
110bbbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb
•
The leading three digits are 110, and the remaining digits can be
either ones or zeroes
•
Note that this scheme reduces the total number of networks
possible by the most significant three bits
Chapter 2
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More About
Address Classes D and E
• Class D addresses always take the form:
1110bbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb
• Class E addresses always take the form:
11110bbb.bbbbbbbb.bbbbbbbb.bbbbbbbbb
• Class D is used for multicast addresses so
that multiple users can “share” a single IP
address and receive the same broadcast
across a network from a single transmission
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Network, Broadcast,
Multicast, and Other
Special IP Addresses
• When an IP packet moves from its sender to
its receiver, the network portion of the
address directs that traffic from the sender’s
network to the receiver’s network
• The only time the host portion of the address
comes into play is when the sender and
receiver both reside on the same physical
network or subnet
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Network, Broadcast,
Multicast, and Other
Special IP Addresses
• The broadcast address represents a network
address that all hosts on a network must read
• Although broadcasts still have some valid uses
on modern networks, they originated in an era
when networks were small and of limited
scope, in which a sort of “all hands on deck”
message represented a convenient way to ask
for services when a specific server could not be
explicitly identified
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Broadcast Packet Structures
• IP broadcast
packets have
two
destination
address
fields—one
Data Link layer
destination
address field,
and one
destination
network
address field
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Multicast Packet
and Address Structures
• When a host uses a service that employs a
multicast address (such as the 224. 0.0.9 address
used for RIPv2 router updates), it registers itself
to “listen” on that address, as well as on its own
unique host address
• That host must also inform its IP gateway (the
router or other device that will forward traffic to
the host’s physical network) that it is registering
for this service so that device will forward such
multicast traffic to that network (otherwise, it will
never appear there)
Chapter 2
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Multicast Packet
and Address Structures
• The Internet
Corporation for
Assigned Names and
Numbers (ICANN)
allocates multicast
addresses on a
controlled basis
• Formerly, addresses
were under the
auspices of IANA, the
Internet Assigned
Numbers Authority
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Multicast Packet
and Address Structures
•
The Data Link layer address 0x01-00-5E-00-05 is
obtained with the following calculation:
1. Replace the first byte with the corresponding 3-byte
OUI. In this case, 224 is replaced with 0x00-00-5E
(assigned to IANA)
2. Change the first byte to an odd value (from 0x00 to
0x01)
3. Replace the second through fourth bytes with their
decimal equivalents
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Data Link Layer Address Conversion
Chapter 2
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The Vanishing IP Address Space
• IP addresses were assigned for public use,
they were assigned on a per-network basis
• With the ever-increasing demand for public IP
addresses for Internet access, it should come
as no surprise that, as early as the mid-1990s,
experts began to predict that the Internet
would “run out” of available IP addresses
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The Vanishing IP Address Space
• The causes for concern have abated somewhat, Here’s
why:
– The technocrats at the IETF introduced a new way to carve up
the IP address space—Classless Inter-Domain Routing (CIDR)
– A brisk trade in existing IP network addresses sprung up
during the same time
– RFC 1918 reserves three ranges of IP addresses for private
use—a single Class A (10.0.0.0-10.255.255.255), 16 Class Bs
(172.16.0.0-172.31.255.255), AND 256 Class
Cs (192.168.0.0-192.168.255.255). When used in tandem with a
technology called Network Address Translation (a.k.a NAT),
private IP addresses can help lift the “cap” on public IP
addresses
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Understanding Basic Binary Arithmetic
•
For the purposes of this text, you must master
four different kinds of binary calculations:
1. Converting binary to decimal
2. Converting decimal to binary
3. Understanding how setting increasing numbers of
high-order bits to 1 in 8-bit binary numbers
corresponds to specific decimal numbers
4. Understanding how setting increasing low-order bits to
1 in 8-bit binary numbers corresponds to specific
decimal numbers
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Converting Decimal to Binary
• Simply divide the number by two, write the
remainder (which must be 0 or 1), then write the
dividend, and repeat until the dividend is zero
• To produce the binary number that corresponds
to 125, you write the digits starting with the last
remainder value, and work your way up:
1111101
• The alternate approach to convert the number
depends on what mathematicians like to call a
“step function”
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Converting Binary to Decimal
•
This is extremely easy, if you know your powers of
two
•
Follow these steps, using 11011011 as the example
number:
1. Count the total number of digits in the number (11011011 has
eight digits)
2. Subtract one from the total (8-1 = 7). That is the power of two
to associate with the highest exponent for two in the
exponential notation for that number
3. Convert to exponential notation, using all the digits as
multipliers
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High-Order Bit Patterns
• In an 8-bit number, there’s little or no value
in blocking less than two bits, or more
than six bits, so the bit patterns you care
about most appear in the second through
the sixth positions in the list of
possibilities shown on page 68 of the
textbook
Chapter 2
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Low-Order Bit Patterns
• Here, we stand the previous example on
its head, and start counting up through
the bit positions in 8-bit numbers from
right to left, adding ones as we increment
• Note that each of these numbers is the
same as two raised to the power of the
number of bits showing, minus one
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Of IP Networks, Subnets, and Masks
• If two network interfaces are on the same
physical network, they can communicate
directly with one another at the MAC layer
• In fact, each of the three primary IP
address classes—namely A, B, and C—
also has an associated default subnet
mask
Chapter 2
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IP Subnets and Supernets
• A subnet mask is a special bit pattern that
“blocks off” the network portion of an IP
address with an all-ones pattern
• The reason why concepts like subnets and
supernets are important for TCP/IP networks
is because each of these ideas refers to a
single “local neighborhood” on such a
network, seen from a routing perspective
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IP Subnets and Supernets
• Thus, a subnet mask that is larger than the
default mask for the address in use divides a
single network IP address into multiple
subnetworks
• The network prefix identifies the number of bits in
the IP address, counting from the left that
represents the actual network address itself, and
the additional two bits of subnetting represent the
bits that were borrowed from the host portion of
that IP address to extend the network portion
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IP Subnets and Supernets
• The entire network address, including the
network prefix and the subnetting bits, is
called the extended network prefix
• This activity of stealing bits from the host
portion of further subdivide the network
portion of an address is called subnetting a
network address, or subnetting
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IP Subnets and Supernets
• When a computer on one subnet wishes to
communicate with a computer on another subnet,
traffic must be forwarded from the sender to a
nearby IP gateway to send the message on its
way from one subnet to another
• Supernetting takes the opposite approach: by
combining contiguous network addresses, it
steals bits from the network portion and uses
them to create a single, larger contiguous
address space for host addresses
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Calculating Subnet Masks
• The simplest form of subnet masking uses a
technique called constant-length subnet masking
(CLSM), in which each subnet includes the same
number of stations and represents a simple division
of the address space made available by subnetting
into multiple equal segments
• Another form of subnet masking uses a technique
called variable-length subnet masking (VLSM) and
permits a single address to be subdivided into
multiple subnets, in which subnets need not all be the
same size
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Designing a
Constant-Length Subnet Mask
• To design a CLSM subnet mask, in which
each portion of the network has the same
number of addresses, follow the steps
outlined on pages 71 and 72 of the
textbook
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Another ConstantLength Subnet Mask Example
• Let’s pick a more ambitious design this
time, which shows how subnet masks or
host addresses can extend across
multiple octets
• Remember, the whole purpose of this
exercise is to compare the number of
hosts needed for each subnet to the
number you just calculated
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Designing a
Variable-Length Subnet Mask
• Designing and configuring variable-length subnet
masks is more time-consuming and difficult, but
makes much better use of available address space
• But let’s suppose that over half the subnets at PU
require support for 30 devices or less
• VSLM was devised to cover this contingency
• VSLM makes it possible to create routing hierarchies
and limit traffic on the backbone by making sure that
smaller subnet address spaces can access the
resources they need as efficiently as possible
Chapter 2
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Designing a
Variable-Length Subnet Mask
• You must:
– Analyze the requirements for individual subnets
– Aggregate those requirements by their relationships to the
nearest power of two that is at least two greater than the
number of such subnets required
– Use the subnets that require the largest number of devices to
decide the minimum size of the subnet mask
– Aggregate subnets that require smaller numbers of hosts
within address spaces defined by the largest subdivisions
– Define a VLSM scheme that provides the necessary number of
subnets of each size to fit its intended use best by aggregating
subnets large and small to create the most efficient network
traffic patterns
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Calculating Supernets
• Supernets “steal” bits from the network
portion of an IP address to “lend” those
bits to the host part
• As part of how they work, supernets
permit multiple IP network addresses to
be combined and make them function
together as if they represent a single
logical network
Chapter 2
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Classless Inter-Domain Routing (CIDR)
• CIDR gets its name from the notion that it
ignores the traditional A, B, and C class
designations for IP addresses, and can
therefore set the network-host ID
boundary wherever it wants to, in a way
that simplifies routing across the resulting
IP address spaces
Chapter 2
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Classless Inter-Domain Routing (CIDR)
• Creating a CIDR address is subject to the
following limitations:
– All the addresses in the CIDR address must be
contiguous
– When address aggregation occurs, CIDR address blocks
work best when they come in sets that are greater than
one, and equal to some lower-order bit pattern that
corresponds to all ones
– CIDR addresses are commonly applied to Class C
addresses
– To use a CIDR address on any network, all routers in the
routing domain must “understand” CIDR notation
Chapter 2
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Public Versus Private IP Addresses
• The private IP address ranges may be expressed in
the form of IP network addresses, as shown in Table
2-4
• Private IP addresses have one other noteworthy
limitation
Chapter 2
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Public Versus Private IP Addresses
• Some IP services require what’s called a secure
end-to-end connection—IP traffic must be able to
move in encrypted form between the sender and
receiver without intermediate translation
• Most organizations need public IP addresses
only for two classes of equipment:
– Devices that permit organizations to attach networks to
the Internet
– Servers that are designed to be accessible to the
Internet
Chapter 2
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Managing Access
to IP Address Information
• Although use of private IP addresses mandates NAT or a
similar address substitutions or masquerade capability,
some organizations elect to use address substitutions or
masquerade even when they use perfectly valid public IP
addresses on their internal networks
• Proxy servers can provide what is sometimes called
reverse proxying
• This permits the proxy server to front for servers inside the
boundary by advertising only the proxy server’s address to
the outside world, and then forwarding only legitimate
requests for service to internal servers for further
processing
Chapter 2
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Obtaining Public IP Addresses
• Unless you work for an organization that has
possessed its own public IP addresses since the
1980s (or acquired such addresses through
merger or acquisition), it’s highly likely that
whatever public IP addresses your organization
uses were issued by the very same ISP who
provides your organization with Internet access
• Because all devices accessible to the Internet
must have public IP addresses, changing
providers often means going through a tedious
exercise called IP renumbering
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IP Addressing Schemes
• To the uninitiated, it may appear that all
these IP addresses are randomly
assigned, or perhaps generated
automatically by some computer
somewhere
• A great deal of thought has gone into the
strategy for allocating IP addresses
around the world
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The Network Space
• There are a number of critical factors that typically
constrain IP addressing schemes, and we look at these in
two groups
• The first group of constraints determines the number and
size of networks
• These are:
– 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
• In most routers, the Layer 3 routing decisions are
typically made by software, so it’s relatively slow
when compared to similar decisions made at
Layer 2 by switches
• This is because switches make their decisions
with specialized hardware known as Application
Specific Integrated Circuits (ASICs)
• A relatively new device known as a layer-3 switch
simply implements the layer-3 logic from the
software into its own ASICs
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The Network Space
• The second group that helps users determine how to
choose which IP addresses go where are design objectives:
– Minimize the size of the routing tables
– Minimize the time required for the network to “converge”
– Maximize flexibility and facilitate management and
troubleshooting
• We already defined a number of networks necessary, so
how do we reduce the number of routes in the routing
table?
The answer is called route aggregation, or summary
addresses
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The Host Space
• Now that you understand some of the factors
involved in numbering the networks, let’s take a
brief look at assigning IP addresses to hosts
• The advantage of a well-thought-out host
naming strategy are a more flexible
environment, and one that is easier to support
• You can easily identify devices by their IP
addresses, regardless of which office they’re in
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Summary
• IP addresses provide the foundation for
identifying individual network interfaces (and
therefore computers or other devices as well) on
TCP/IP networks
• IP addresses come in five classes named A
through E
• Classes A through C use the IPv4 32-bit address
to establish different break points between the
network and host portions of such network
addresses
Chapter 2
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Summary
• Understanding binary arithmetic is essential to
knowing how to deal with IP addresses, particularly
when working with subnet masks
• To help ease address scarcity, the IETF created a form
of classless addressing called Classless Inter-Domain
Routing (CIDR) that permits the network-host
boundary to fall away from octet boundaries
• Likewise, to make best use of IP network addresses, a
technique called subnetting permits additional bits to
be taken from the host portion of a network
Chapter 2
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Summary
• Several techniques exist to hide internal network IP
addresses from outside view, including address
masquerading and address substitution
• Within the Class A, B, and C IP address ranges, the
IETF has reserved private IP addresses or address
ranges
• When it comes to obtaining public IP addresses, the
Internet Corporation for Assigned Names and
Numbers (ICANN), previously the Internet Assigned
Numbers Authority, or IANA, handled this task) is the
ultimate authority
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