Transcript IP`s and Masks - La Salle University

IP Addresses
Based Computer Networks and
Internets (Comer)
CSIT 320 (Blum)
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IP Address
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The logical (and thus software) destination is
denoted by an IP Address.
The IP Address provides homogeneity over
diverse networks.
The IP Address allows a host to change
hardware (and thus hardware address) and
yet still be found at the software level.
Also IP Addresses are assigned in a more
logical manner, which can facilitate routing.
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Prefix/Suffix
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The routing is facilitated by the fact that an
IP Address is hierarchical.
It consists of two parts:
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The first part of the address (starting on the
left) is known as the prefix and it identifies the
host’s network (the group of computers it
belongs to).
The second part of the address is known as the
suffix and it identifies the individual computer
(node) within the above specified network.
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IP Addressing Scheme
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In IP(v4) each host is assigned a
unique 32-bit number which is the
address for the host.
To transmit on a TCP/IP internet, a host
must know its own IP address as well
as that of the destination.
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We’ll qualify the above statement later.
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Dotted Decimal Notation
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Each 8-bit section (known as a byte or octet) of
the 32-bit number [IP(v4)] is expressed as a
decimal value with periods between them.
The combination of eight bits can be in 256=28
states, which are expressed as the numbers 0
through 255.
The range of valid addresses which can be
assigned is 0.0.0.0 to 255.255.255.255, which
barring various reserved addresses is
4,294,967,296=232
The La Salle network is 139.84.0.0 , the computer
www.lasalle.edu is 139.84.10.250
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Uniqueness
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Each computer on an internet (or on the
Internet) must have a unique address.
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Two hosts on the same internet can have
different network portions and the same node
portion.
Two hosts on the same internet can have the
same network portion and different node
portions.
Two hosts on the same internet can have
different network portions and different node
portions.
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Address Classes
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The IP address is not divided into equal
halves with one half for the network portion,
the other for the node potion
The addressing scheme tries to accommodate
for the fact that
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Some networks (not many) will contain a vast
number of hosts
While other networks (very many) will contain a
more modest number of hosts
Thus the IP Class system was developed.
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The IP Class System
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Originally the IP(v4) Addresses were broken
into 5 classes: A through E.
The dividing line between network portion
and node portion of the IP address differed
from class to class.
The first four bits on the left identify the class
to which an address belongs.
A, B and C are the primary classes for the
addressing, D and E were reserved.
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Reserved Class
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Class D is used for multicasting. Class
D addresses begin with the first four
bits 1110.
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If a set of hosts uses multicasting, they
agree to share the multicast address.
When a message is transmitted to the
multicast address, each host in the group
makes a copy.
Class E is reserved for future use. Class
E addresses begin with the first four bits
1111.
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Figure from Comer’s book
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Class A
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Class A is self-identified by the leftmost bit
being a 0.
Class A uses the first octet from the left to
identify the network and the rest to identity
the nodes.
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It has 7 bits (first octet minus first bit used to
indicate class A) to identify networks, so there can
be 128 = 27 Class A networks.
It has 24 bits (the last three octets) to identify
nodes, so there can be 16777216=224 nodes on a
Class A network (almost).
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Reserved Addresses
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Actually the node addresses consisting
of all 1’s and all 0’s are reserved, so
the number of Class A nodes is actually
16,777,214=224 – 2
All 0’s (in the suffix) is reserved to refer
to the network itself.
All 1’s (in the suffix) is used to
broadcast on the network.
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Class B
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Class B is self-identified by the first two bits
being a 10.
Class B uses the first two octets from the left
to identify the network and the rest to
identity the nodes
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It has 14 bits (first two octet minus first two bits
used to indicate class B) to identify networks, so
there can be 16384 = 214 Class B networks.
It has 16 bits (the last two octets) to identify
nodes, so there can be 65534=216 –2 nodes on a
Class B network.
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Class C
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Class C is self-identified by the first three bits
being a 110.
Class C uses the first three octets from the
left to identify the network and the remaining
one to identity the nodes.
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It has 21 bits (first three octet minus first three bits
used to indicate class C) to identify networks, so
there can be 2097152 = 221 Class C networks.
It has 8 bits (the last octet) to identify nodes, so
there can be 254 =28 –2 nodes on a Class C
network.
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These are all off by 2 because it is neglected
by node addresses (suffixes) reserved for the
network and broadcasting
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Computing Address Class
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In Class A, the first octet starts with a 0,
thus the smallest number is
0
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0
0
0
0
0
0
And the largest number (in Class A) is
0
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0
1
1
1
1
1
1
1
So in decimal-dot notation, Class A
addresses start with a number between 0
and 127
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Computing Address Class
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In Class B, the first octet starts with a 10,
thus the smallest number is
1
0
0
0
0
0
0
0
And the largest number is
1
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0
1
1
1
1
1
1
So in decimal-dot notation, Class B addresses
start with a number between 128 and 191
www.lasalle.edu (139.84.10.250) is Class B
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Computing Address Class
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In Class C, the first octet starts with a 110,
thus the smallest number is
1
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0
0
0
0
0
0
1
1
1
And the largest number is
1
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1
1
0
1
1
So in decimal-dot notation, Class C addresses
start with a number between 192 and 223
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subnet
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A Class B network (like LaSalle’s) can have
65534 hosts.
To manage traffic within the network, it is
useful to break the network into subnetworks (subnets).
On TCP/IP networks, subnets are defined as
all devices whose IP addresses have the same
prefix.
But Class B has a two-octet prefix, so
wouldn’t all 65534 of the hosts be on the
same subnet?
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Subnet mask
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Dividing a network into subnets is useful for
both security and performance reasons.
The division of the prefix and suffix portions
was made more flexible by using subnet
masks.
In addition to setting an IP address, one also
sets a subnet mask which specifies which
portion of the address is used to identify the
network and which portion is used to identify
the hosts within a network.
The 1’s in the subnet mask correspond to
the network part, the 0’s correspond to the
host part.
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Subnet mask
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By convention, the bits for the network
address are all set to 1
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It would also work if the bits were set exactly
the same as in the network address (prefix).
A typical subnet mask looks like
11111111.11111111.11110000.0000000
0.
One extracts the subnet address by
performing a bitwise AND operation on
the mask and the IP address.
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CIDR
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The use of subnet masking instead of
classes sometimes goes by the name
Classless Inter-Domain Routing (CIDR).
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Special IP Addresses
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Network Addresses: IP reserves the host (suffix)
address of 0’s for a network
Direct Broadcast: IP reserves the host (suffix)
address of 1’s for broadcasting within the network
Limited Broadcast: Used during system startup by a
computer that does not know its IP address. The
entire prefix and suffix are assigned all 1’s for the
local network.
This Computer: Used by a computer to define its
address. The computer needs to send or receive
packets to determine its address on the network.
This happens during startup. IP reserves the
address of all zeros to mean the initial host
computer.
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LoopBack
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Used to test network applications. Can test
computer-to-computer applications using one
computer by forcing a packet down through the
protocol stack by using the loopback address.
The application uses the loopback address to send
the data to “another” application which is running
on the same machine.
During loopback testing, no packets leave the
computer.
The network prefix of 127 is reserved for loopback.
Any suffix is used.
 (Reduces the number of Class A networks.)
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Ping localhost
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Berkeley Broadcast Address
Form
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TCP/IP was distributed with a version of
Unix put out by Berkeley.
In that version of TCP/IP, all 0’s in the
suffix is used for as broadcasting
(instead of all 1’s).
Many people used this version, so now
there are standard broadcasts and
Berkeley broadcasts.
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Routers and IP Addressing
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Routers are also given IP
addresses, actually they are given 2
or more addresses since a router
connects to more than one physical
network.
To make network administrator’s
lives easier, they often assign the
same suffix to the various router
addresses. (The prefixes must of
course be different.)
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Multi-Homing    
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Sometimes computers also have
multiple IP addresses (and multiple NIC
cards).
If a host computer connects to multiple
networks, it is called multi-homed.
This may increase reliability and
performance, since it is still networked
if only one of the networks goes down.
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References
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Comer, Computer Networks and
Internets
http://www.whatis.com
http://www.webopdeia.com
http://public.pacbell.net/dedicated/cidr.
html
http://www.netsol.com/cgibin/whois/whois
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