Download Introducing TCP/IP

IP Addressing
“If we all did the things we are capable of doing,
we would literally astound ourselves”
- Thomas Alva Edison, 1847-1931
Chapter 2
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Objectives
Chapter 2
• Recognize and describe the various IP address classes
from A to E, and explain how they’re composed and
used
• Describe the IPv4 address limitations, and how
techniques like Classless Inter-Domain Routing (CIDR)
and use of private IP addresses with Network Address
Translation (NAT) ease those limitations
• Define the terms subnet and supernet, and apply
subnetting and supernetting concepts in solving specific
network design problems
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Chapter 2
IP Addressing Basics
• Different addressing schemes:
– Symbolic (eg: www.bcit.ca)
– Logical numeric (eg: 172.16.1.10)
– Physical numeric (eg: 6 byte MAC addresses)
• Symbolic addresses are easier to remember than a numeric
address such as 199.95.72.8
• Physical numeric addresses are MAC layer addresses associated
with the Data Link layer (of the OSI Reference model)
• Logical numeric addresses are IP addresses associated with the
Network layer
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Chapter 2
IP Addressing
 IPv4 uses 32-bit addresses, commonly represented in dotted
decimal notation.
Eg: 11000000 00001100 00001010 00000101 (in binary)
192
12
10
5
(each octet in decimal)
Written as: 192.12.10.5
(in dotted decimal notation)
 Classful Addresses
 Address range is divided into 5 classes (A to E)
 Each address has two parts:
• Network address (Net id) and Host address (Host id)
• A two-level hierarchy
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Chapter 2
Classful Addresses
Class A
1
0
Net id
Class B
1 0
Class C
1 1 0
Class D
1 1 1 0
Class E
1 1 1 1 0
Host id (24 bits)
Net id
Host id (16 bits)
Net id
Host id (8 bits)
Multicast group id
Reserved for future use
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Chapter 2
Address ranges for different classes
Class
A
B
C
D
E
1.0.0.0
128.0.0.0
192.0.0.0
224.0.0.0
240.0.0.0
Range
to 126.255.255.255
to 191.255.255.255
to 223.255.255.255
to 239.255.255.255
to 255.255.255.255
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Chapter 2
Classful Addresses
• Class A - only ~125 networks possible
– Each network can support 16,777,214 hosts
(2^24 - 2)
– 0.0.0.0 is not assigned to a specific network
– The address range 10.x.x.x (x: 0-255) is
reserved for private network use (as per RFC
1918)
– 127.x.x.x (x: 0-255) is reserved for loopback
testing
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Chapter 2
Classful Addresses
• Class B - for moderate to large networks
– Each network can support 65,534 hosts (2^16 2)
– The address range 172.16.0.0 to 172.31.255.255
is reserved for private use
• Class C - for small networks
– Each network can support 254 hosts (2^8 - 2)
– The address range 192.168.0.0 -192.168.255.255
is reserved for private use
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Chapter 2
Types of Addresses
• Unicast: data sent to a single host (or, an interface on a
machine)
• Broadcast: sent to all hosts on a network
– Directed broadcast - host id with all 1’s
• Eg: A packet sent to 190.10.255.255 is received by
all hosts on the network 190.10.0.0
• Routers may forward these broadcast packets
– Limited broadcast - 255.255.255.255
• Never forwarded by a router
• Multicast: sent to a set of hosts that belong to a
“multicast” group
• Host id with all 0’s is not assigned as a host address, but
identifies the network.
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Chapter 2
Subnetting
• A network can be divided into sub-networks
internally, by dividing the host portion of an
IP address into a subnet id and a host id
within the subnetwork (a three-level
hierarchy)
• This activity of stealing bits from the host
portion to further subdivide the network
portion of an address is called subnetting a
network address, or subnetting
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Chapter 2
Subnet Mask
• A 32-bit subnet mask identifies the network
and subnet bits in an IP address
• If a bit value is 1 in the subnet mask, the
corresponding bit in the IP address is
considered part of the network address
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Chapter 2
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 hosts 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 network address to be subdivided
into multiple subnets, in which subnets need not all
be the same size
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Chapter 2
Subnetting Example 1:
• An large organization is assigned with the
network address 190.10.0.0/16. It needs to
support about 150 subnets for different
locations. In each subnet, it needs to support
about 200 hosts.
• As the first step, decide the number of bits
needed from host bits to represent the
subnet ID.
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Chapter 2
Subnetting Example 1:
• Subnetting the network 190.10.0.0 by using
8 bits of the 16 host id bits
–
–
–
–
Subnet mask: 255.255.255.0
Possible subnets: 2^8 => 256
Possible hosts per subnet: 2^8 - 2 => 254
Addresses of subnetworks:
•
•
•
•
190.10.0.0 (Subnet #0)
190.10.1.0 (Subnet #1)
….
190.10.255.0 (Subnet #255)
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Chapter 2
Subnetting Example 1 ...
• For Subnet #0:
– A typical host address is 190.10.0.x where x = 1 to
254 (eg: 190.10.0.5), with a subnet mask of
255.255.255.0
– Also written as: 190.10.0.5/24 (without having to
write the subnet mask) - Binary Count notation
– “24” identifies the number of contiguous 1 bits in
the subnet mask and is called the “length of the
Extended-Network-Prefix”
– Directed broadcast addresses of subnet #0:
• 190.10.0.255
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Subnetting Example 2:
• An organization is assigned with network address
193.1.1.0/24. It needs to define 6 subnets for internal
departments. The largest subnet need to support 25
hosts.
• Step 1: Determine the no. of bits needed from the
host id bits (8 in this case) to define 6 subnets
– 3 bits => 8 subnets (2 extra for future expansion)
• Step 2: Determine whether the remaining host id bits
(5 in this case) is sufficient for max. hosts needed per
subnet
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Chapter 2
Subnetting Example 2 ...
• Step 2 continued …
– 5 bits => 2^5 - 2 => 30 hosts per subnet
• Subnet mask for each subnet:
– 11111111 11111111 11111111 11100000
– 255.255.255.224
• Extended network prefix for each subnet: /27
• Network addresses:
– Base network: 193.1.1.0/24
– Subnet #0: 193.1.1.0/27
– Subnet #7: 193.1.1.224/27
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Chapter 2
Subnetting Example 2 ...
• Valid host addresses for Subnet #2:
–
–
–
–
–
–
–
–
Subnet#2: 11000001.00000001.00000001.010 00000 = 193.1.1.64/27
Host #1: 11000001.00000001.00000001.010 00001 = 193.1.1.65/27
Host #2: 11000001.00000001.00000001.010 00010 = 193.1.1.66/27
Host #3: 11000001.00000001.00000001.010 00011 = 193.1.1.67/27
….
Host#16: 11000001.00000001.00000001.010 10000 = 193.1.1.80/27
….
Host#30: 11000001.00000001.00000001.010 11110 = 193.1.1.94/27
• Broadcast address for each subnet:
– Host id with all 1’s
– For Subnet #2 above:
• 11000001.00000001.00000001.010 11111 = 193.1.1.95/27
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Chapter 2
More Examples ...
• A host IP address is 193.27.100.110/26.
Determine:
– the subnet address
– directed broadcast address for the subnet
– maximum number of possible hosts on the
subnet
– maximum number of possible subnets
(assuming constant length subnet masking)
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Chapter 2
To find the subnet address ...
• When a host IP address is given, to find the subnet
address:
– convert the dotted decimal address to binary notation
(not necessary to convert decimal digits containing
solely network bits to binary)
– identify the host bits in the IP address, using the subnet
mask or the extended network prefix
– set all these host bits to zero
– convert the resulting binary number back to dotted
decimal notation
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Chapter 2
To find the subnet address ...
• In 193.27.100.110/26, there are 26 network bits (26
most significant bits) and 6 (32-26) host bits
• This means, the decimal digit 110 contains 2 network
bits (2 most significant bits) and 6 host bits (6 least
significant bits)
• decimal 110 => binary 01 101110
• Host bits are: 101110
• Setting host bits to 0 => 01 000000 => 64 (decimal)
• Therefore, subnet address = 193.27.100.64/26
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Chapter 2
To find the broadcast address ...
• When a host IP address is given, to find the
broadcast address:
– convert the dotted decimal address to binary notation
(not necessary to convert decimal digits containing
solely network bits to binary)
– identify the host bits in the IP address, using the subnet
mask or the extended network prefix
– set all these host bits to 1
– convert the resulting binary number back to dotted
decimal notation
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Chapter 2
To find the broadcast address ...
• As discussed previously, host bits are: 101110
• Setting host bits to 1 => 01 111111
=> 127 (decimal)
• Therefore, broadcast address = 193.27.100.127/26
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Chapter 2
To find the maximum number of
possible hosts in a subnet ...
• Number of host bits = 6 (32-26)
• Max. possible addresses per subnet = 2^6 = 64
• As host bits with all 0’s and all 1’s are not valid
host addresses, max. number of hosts possible
= 64-2 => 62
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Chapter 2
To find the maximum number of
subnets ...
• Number of subnet bits = 26 - 24 => 2
(where: 26 = total number of network bits
24 = default network bits in the given
Class C address)
• Max. possible subnets = 2^2 = 4
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Chapter 2
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Chapter 2
Variable Length Subnet Masks (VLSM)
• A limitation of having only a single subnet mask across
a given network-prefix is that once the mask is selected,
it locks the organization into a fixed number of fixedsized subnets.
• In Subnetting Example 1 (subnetting 190.10.0.0 using 8
bits of the host id), there are 256 possible subnets with
254 hosts each.
– If a small subnet needs only a max. of 10 hosts, this
wastes IP addresses
• A solution is to allow a subnetted network to use more
than one subnet mask (RFC 1009)
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Chapter 2
VLSM Example:
• An organization is assigned the network number
140.25.0.0/16. It plans to divide the address space
into 16 equal sized blocks (subnets 0-15), and
then to sub-divide subnet #14 into 16 equal-sized
blocks.
• Using 4 bits for subnet id, 16 subnets of the
140.25.0.0/16 address block are:
Base net:
Subnet #0:
Subnet #1:
….
Subnet #14:
Subnet #15:
10001100.00011001.00000000.00000000
10001100.00011001.00000000.00000000
10001100.00011001.00010000.00000000
= 140.25.0.0/16
= 140.25.0.0/20
= 140.25.16.0/20
10001100.00011001.11100000.00000000
10001100.00011001.11110000.00000000
= 140.25.224.0/20
= 140.25.240.0/20
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Chapter 2
VLSM Example ...
• Using 4 more bits for sub-subnet id, 16 subsubnets of Subnet #14 are:
Subnet #14:
10001100.00011001.11100000.00000000 = 140.25.224.0/20
Subnet #14-0: 10001100.00011001.11100000.00000000 = 140.25.224.0/24
Subnet #14-1: 10001100.00011001.11100001.00000000 = 140.25.225.0/24
….
Subnet #14-14: 10001100.00011001.11101110.00000000 = 140.25.238.0/24
Subnet #14-15: 10001100.00011001.11101111.00000000 = 140.25.239.0/24
• Host addresses for Subnet #14-1:
Host #1:
10001100.00011001.11100001.00000001
Host #2:
10001100.00011001.11100001.00000010
….
Host #254: 10001100.00011001.11100001.11111110
= 140.25.225.1/24
= 140.25.225.2/24
= 140.25.225.254/24
• Broadcast address for Subnet #14-1= 140.25.225.255
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Chapter 2
The Vanishing IP Address Space
• Interim solutions for IPv4 address depletion
problem:
– IETF introduced a new way to carve up the IP address
space—Classless Inter-Domain Routing (CIDR)
– 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 together
with Network Address Translation (a.k.a NAT), private IP
addresses can help lift the “cap” on public IP addresses
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Chapter 2
Classless Inter-Domain Routing (CIDR)
• Abandons the rigid address classes to eliminate the
inefficiency in classful addressing
• CIDR 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.
• To use a CIDR address on any network, all routers
in the routing domain must “understand” CIDR
notation
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Chapter 2
Classless Inter-Domain Routing (CIDR)
• Allows more efficient aggregation of routing info
– Route Aggregation: Use of a single entry in a routing
table to represent address space of several networks
– Reduces the size of routing tables in routers
• Allows Supernetting
– Using contiguous blocks of Class C addresses to
simulate a single, large address space
• Documented in RFCs 1517 to 1520
• Eg: 192.125.61.8/20 identifies a network with a 20-bit
network prefix
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Supernets
Chapter 2
• Supernetting takes the opposite approach to subnetting:
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
• Example: An organization has the following contiguous
Class C addresses
212.56.132.0/24 11010100 00111000 10000100 00000000
212.56.133.0/24 11010100 00111000 10000101 00000000
212.56.134.0/24 11010100 00111000 10000110 00000000
212.56.135.0/24 11010100 00111000 10000111 00000000
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Supernets
Chapter 2
• The common prefix for all the 4 addresses is:
11010100 00111000 100001
• They can be aggregated as: 212.56.132.0 / 22
• In the Supernet, the network ID has 22 bits and the host
ID has 10 bits
• The network address of supernet: 212.56.132.0/22
• The broadcast address of supernet: 212.56.135.255/22
• Valid Host addresses:
212.56.132.1/22 - 212.56.135.254/22
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Chapter 2
Summary
• IP addresses allow identifying individual network
interfaces (and therefore computers or other
devices as well) on TCP/IP networks
• With Classful addressing, 5 address classes (A to
E) are defined
• Classes A through C are assigned to individual
hosts and consists of network ID and host ID
portions
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Chapter 2
Summary
• 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 basically anywhere
• Subnetting divides an assigned address space
into smaller groups (subnetworks) by using
bits from the host portion to form a
subnetwork ID
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Chapter 2
Summary
• Within the Class A, B, and C IP address ranges, the
IETF has reserved private IP address ranges
• With CIDR, Supernetting is possible. Supernetting
allows borrowing bits from the network portion
(opposite of subnetting) to be used as host
addresses, to form a “Supernet” by combining
contiguous Class C addresses
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Chapter 2
References
• RFC 1878, Variable Length Subnet Table For
IPv4, Dec.1995
• http://www.mcmcse.com/articles/subnetting.
shtml (on Subnetting Confusion)
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