Given an IP address and subnet mask, you can
identify the subnet address, broadcast address, and
first and last usable addresses within a subnet as
follows:
1. Write down the 32-bit address and the subnet
mask below that (174.24.4.176/26 is shown
in the following figure).
2. Draw a vertical line just after the last 1 bit in
the subnet mask.
3. Copy the portion of the IP address to the left
of the line. Place all 1s for the remaining free
spaces to the right. This is the broadcast
address for the subnet.
4. The first and last address can also be found
by placing ...0001 and ...1110, respectively, in
the remaining free spaces.
5. Copy the portion of the IP address to the left
of the line. Place all 0s for the remaining free
spaces to the right. This is the subnet number.
174.24.4.176 1010111000110000000100 10110000 Host
255.255.255.192 1111111111111111111111 11000000 Mask
174.24.4.128 1010111000110000000100 10000000 Subnet
174.24.4.191 1010111000110000000100 10111111 Broadcast
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Subnet Masks
Routers use a subnet mask to determine which
parts of the IP address correspond to the network,
the subnet, and the host. The mask is a 32-bit
number in the same format as the IP address. The
mask is a string of consecutive 1s starting from the
most-significant bits, representing the network ID,
followed by a string of consecutive 0s, representing
the host ID portion of the address bits.
Each address class has a default subnet mask (A =
/8, B = /16, C = /24). The default subnet masks
only the network portion of the address, the effect
of which is no subnetting. With each bit of subnetting
beyond the default, you can create 2n–2 subnets.
The preceding example has 254 subnets, each
with 254 hosts. This counts the address ending
with .0, but not the address ending in .255.
Continuing with the preceding analogy, the subnet
mask tells the network devices how many apartments
are in the building.
Identifying Subnet Addresses
Given an IP address and subnet mask, you can
identify the subnet address, broadcast address, and
first and last usable addresses within a subnet as
follows:
1. Write down the 32-bit address and the subnet
mask below that (174.24.4.176/26 is shown
in the following figure).
2. Draw a vertical line just after the last 1 bit in
the subnet mask.
3. Copy the portion of the IP address to the left
of the line. Place all 1s for the remaining free
spaces to the right. This is the broadcast
address for the subnet.
4. The first and last address can also be found
by placing ...0001 and ...1110, respectively, in
the remaining free spaces.
5. Copy the portion of the IP address to the left
of the line. Place all 0s for the remaining free
spaces to the right. This is the subnet number.
174.24.4.176 1010111000110000000100 10110000 Host
255.255.255.192 1111111111111111111111 11000000 Mask
174.24.4.128 1010111000110000000100 10000000 Subnet
174.24.4.191 1010111000110000000100 10111111 Broadcast
At-a-Glance: IP Addressing
Network
128
10000000
10
00001010
173
10110010
46
00101110
Host
IP
Address
Subnet
Mask
This subnet mask can also be written as "/24", where 24
represents the number of 1s in the subnet mask.
parts of the IP address correspond to the network,
the subnet, and the host. The mask is a 32-bit
number in the same format as the IP address. The
mask is a string of consecutive 1s starting from the
most-significant bits, representing the network ID,
followed by a string of consecutive 0s, representing
the host ID portion of the address bits.
Each address class has a default subnet mask (A =
/8, B = /16, C = /24). The default subnet masks
only the network portion of the address, the effect
of which is no subnetting. With each bit of subnetting
beyond the default, you can create 2n–2 subnets.
The preceding example has 254 subnets, each
with 254 hosts. This counts the address ending
with .0, but not the address ending in .255.
Continuing with the preceding analogy, the subnet
mask tells the network devices how many apartments
are in the building.
Identifying Subnet Addresses
Given an IP address and subnet mask, you can
identify the subnet address, broadcast address, and
first and last usable addresses within a subnet as
follows:
1. Write down the 32-bit address and the subnet
mask below that (174.24.4.176/26 is shown
in the following figure).
2. Draw a vertical line just after the last 1 bit in
the subnet mask.
3. Copy the portion of the IP address to the left
of the line. Place all 1s for the remaining free
spaces to the right. This is the broadcast
address for the subnet.
4. The first and last address can also be found
by placing ...0001 and ...1110, respectively, in
the remaining free spaces.
5. Copy the portion of the IP address to the left
of the line. Place all 0s for the remaining free
spaces to the right. This is the subnet number.
174.24.4.176 1010111000110000000100 10110000 Host
255.255.255.192 1111111111111111111111 11000000 Mask
174.24.4.128 1010111000110000000100 10000000 Subnet
174.24.4.191 1010111000110000000100 10111111 Broadcast
At-a-Glance: IP Addressing
Network
128
10000000
10
00001010
173
10110010
46
00101110
Host
IP
Address
Subnet
Mask
This subnet mask can also be written as "/24", where 24
represents the number of 1s in the subnet mask.
Subnetting
Subnetting is a method of segmenting hosts within
a network and providing additional structure.
Without subnets, an organization operates as a flat
network. These flat topologies result in short routing
tables, but as the network grows, the use of
bandwidth becomes inefficient.
In the figure, a Class B network is flat, with a single
broadcast and collision domain. Collision
domains are explained in more detail in the
Ethernet chapter. For now, just think of them as a
small network segment with a handful of devices.
Adding Layer 2 switches to the network creates
more collision domains but does not control
broadcasts.
In the next figure, the same network has been subdivided
into several segments or subnets. This is
accomplished by using the third octet (part of the
host address space for a Class B network) to segment
the network. Note that the outside world
sees this network the same as in the previous figure.
Subnetting is a bit complex at first pass. Think of
it like a street address. For a house, the street
address may provide the needed addressability to
reach all the house’s occupants. Now consider an
apartment building. The street address only gets
you to the right building. You need to know in
which apartment the occupant you are seeking
resides. In this crude example, the apartment number
acts a bit like a subnet.
a network and providing additional structure.
Without subnets, an organization operates as a flat
network. These flat topologies result in short routing
tables, but as the network grows, the use of
bandwidth becomes inefficient.
In the figure, a Class B network is flat, with a single
broadcast and collision domain. Collision
domains are explained in more detail in the
Ethernet chapter. For now, just think of them as a
small network segment with a handful of devices.
Adding Layer 2 switches to the network creates
more collision domains but does not control
broadcasts.
In the next figure, the same network has been subdivided
into several segments or subnets. This is
accomplished by using the third octet (part of the
host address space for a Class B network) to segment
the network. Note that the outside world
sees this network the same as in the previous figure.
Subnetting is a bit complex at first pass. Think of
it like a street address. For a house, the street
address may provide the needed addressability to
reach all the house’s occupants. Now consider an
apartment building. The street address only gets
you to the right building. You need to know in
which apartment the occupant you are seeking
resides. In this crude example, the apartment number
acts a bit like a subnet.
Address Classes
When the IP address scheme was developed, only
the first octet was used to identify the network
portion of the address. At the time it was assumed
that 254 networks would be more than enough to
cover the research groups and universities using
this protocol. As usage grew, however, it became
clear that more network designations would be
needed (each with fewer hosts). This issue led to
the development of address classes.
Addresses are segmented into five classes (A
through E). Classes A, B, and C are the most common.
Class A has 8 network bits and 24 host bits.
Class B has 16 network bits and 16 host bits, and
Class C has 24 network bits and 8 host bits. This
scheme was based on the assumption that there
would be many more small networks (each with
fewer endpoints) than large networks in the world.
Class D is used for multicast, and Class E is
reserved for research. The following table breaks
down the three main classes. Note that the Class A
address starting with 127 is reserved.
the first octet was used to identify the network
portion of the address. At the time it was assumed
that 254 networks would be more than enough to
cover the research groups and universities using
this protocol. As usage grew, however, it became
clear that more network designations would be
needed (each with fewer hosts). This issue led to
the development of address classes.
Addresses are segmented into five classes (A
through E). Classes A, B, and C are the most common.
Class A has 8 network bits and 24 host bits.
Class B has 16 network bits and 16 host bits, and
Class C has 24 network bits and 8 host bits. This
scheme was based on the assumption that there
would be many more small networks (each with
fewer endpoints) than large networks in the world.
Class D is used for multicast, and Class E is
reserved for research. The following table breaks
down the three main classes. Note that the Class A
address starting with 127 is reserved.
Logical Versus Physical
MAC addresses are considered physical addresses
because they are assigned to pieces of hardware by
the manufacturer and cannot be reassigned.
IP addresses are assigned by a network administrator
and have meaning only in a TCP/IP network.
These addresses are used solely for routing purposes
and can be reassigned.
Host and network: Rather than assigning numbers
at random to various endpoints (which would be
extremely difficult to manage), every company and
organization listed on the Internet is given a block
of public address numbers to use. This is accomplished
by using a two-part addressing scheme that
identifies a network and host. This two-part
scheme allows the following:
• All the endpoints within a network share the
same network number.
• The remaining bits identify each host within
that network.
In the figure, the first two octets (128.10) identify
a company with an Internet presence (it’s the
address of the router that accesses the Internet).
All computers and servers within the company’s
network share the same network address. The next
two octets identify a specific endpoint (computer,
server, printer, and so on). In this example the
company has 65,536 addresses it can assign (16
bits, or 216). Therefore, all devices in this network
would have an address between 128.10.0.1 and
128.10.255.255.
because they are assigned to pieces of hardware by
the manufacturer and cannot be reassigned.
IP addresses are assigned by a network administrator
and have meaning only in a TCP/IP network.
These addresses are used solely for routing purposes
and can be reassigned.
Host and network: Rather than assigning numbers
at random to various endpoints (which would be
extremely difficult to manage), every company and
organization listed on the Internet is given a block
of public address numbers to use. This is accomplished
by using a two-part addressing scheme that
identifies a network and host. This two-part
scheme allows the following:
• All the endpoints within a network share the
same network number.
• The remaining bits identify each host within
that network.
In the figure, the first two octets (128.10) identify
a company with an Internet presence (it’s the
address of the router that accesses the Internet).
All computers and servers within the company’s
network share the same network address. The next
two octets identify a specific endpoint (computer,
server, printer, and so on). In this example the
company has 65,536 addresses it can assign (16
bits, or 216). Therefore, all devices in this network
would have an address between 128.10.0.1 and
128.10.255.255.
What Problems Need to Be Solved?
Each IP address is a 32-bit number, which means
that there are about 4.3 trillion address combinations.
These addresses must be allocated in a way
that balances the need for administrative and routing
efficiency with the need to retain as many
usable addresses as possible.
Dotted decimal: The most common notation for
describing an IP address is dotted decimal. Dotted
decimal breaks a 32-bit binary number into four
8-bit numbers (represented in decimal form), which
is called an octet. Each octet is separated by a period,
which aids in the organizational scheme to be
discussed. For example, the binary address
00001010100000001011001000101110 can be
represented in dotted decimal as 10.128.178.46.
that there are about 4.3 trillion address combinations.
These addresses must be allocated in a way
that balances the need for administrative and routing
efficiency with the need to retain as many
usable addresses as possible.
Dotted decimal: The most common notation for
describing an IP address is dotted decimal. Dotted
decimal breaks a 32-bit binary number into four
8-bit numbers (represented in decimal form), which
is called an octet. Each octet is separated by a period,
which aids in the organizational scheme to be
discussed. For example, the binary address
00001010100000001011001000101110 can be
represented in dotted decimal as 10.128.178.46.
Why Should I Care About IP Addressing?
Behind every website, Universal Resource Locator
(URL), and computer or other device connected to
the Internet is a number that uniquely identifies
that device. This unique identifier is called an IP
address. These addresses are the key components
of the routing schemes used over the Internet. For
example, if you are downloading a data sheet from
www.cisco.com to your computer, the header of
the packets comprising the document includes both
the host address (in this case, the IP address of
Cisco’s public server) and the destination address
(your PC).
(URL), and computer or other device connected to
the Internet is a number that uniquely identifies
that device. This unique identifier is called an IP
address. These addresses are the key components
of the routing schemes used over the Internet. For
example, if you are downloading a data sheet from
www.cisco.com to your computer, the header of
the packets comprising the document includes both
the host address (in this case, the IP address of
Cisco’s public server) and the destination address
(your PC).
Port Numbers
TCP and UDP can send data from several upperlayer
applications on the same datagram. Port
numbers (also called socket numbers) are used to
keep track of different conversations crossing the
network at any given time. Some of the more wellknown
port numbers are controlled by the Internet
Assigned Numbers Authority (IANA). For example,
Telnet is always defined by port 23.
Applications that do not use well-known port
numbers have numbers randomly assigned from a
specific range.
The use of port numbers is what allows you to
watch streaming video on your computer while
checking e-mails and downloading documents
from a web page all at the same time. All three
may use TCP/IP, but use of a port number allows
the applications to distinguish which are video and
which are e-mail packets.
applications on the same datagram. Port
numbers (also called socket numbers) are used to
keep track of different conversations crossing the
network at any given time. Some of the more wellknown
port numbers are controlled by the Internet
Assigned Numbers Authority (IANA). For example,
Telnet is always defined by port 23.
Applications that do not use well-known port
numbers have numbers randomly assigned from a
specific range.
The use of port numbers is what allows you to
watch streaming video on your computer while
checking e-mails and downloading documents
from a web page all at the same time. All three
may use TCP/IP, but use of a port number allows
the applications to distinguish which are video and
which are e-mail packets.
UDP
UDP is a connectionless, unreliable Layer 4 protocol.
Unreliable in this sense means that the protocol
does not ensure that every packet will reach its
destination. UDP is used for applications that provide
their own error recovery process or when
retransmission does not make sense. UDP is simple
and efficient, trading reliability for speed.
Why not resend? It may not be obvious why you
would not resend dropped packets if you had the
option to do so. However, real-time applications
such as voice and video could be disrupted by
receiving old packets out of order. For example,
suppose a packet containing a portion of speech is
received 2 seconds later than the rest of the conversation.
Playing the sound out into the earpiece
probably will sound like poor audio quality to the
user, because the user is listening further into the
conversation. In these cases, the application usually
can conceal the dropped packets from the end
user so long as they account for a small percentage
of the total.
Unreliable in this sense means that the protocol
does not ensure that every packet will reach its
destination. UDP is used for applications that provide
their own error recovery process or when
retransmission does not make sense. UDP is simple
and efficient, trading reliability for speed.
Why not resend? It may not be obvious why you
would not resend dropped packets if you had the
option to do so. However, real-time applications
such as voice and video could be disrupted by
receiving old packets out of order. For example,
suppose a packet containing a portion of speech is
received 2 seconds later than the rest of the conversation.
Playing the sound out into the earpiece
probably will sound like poor audio quality to the
user, because the user is listening further into the
conversation. In these cases, the application usually
can conceal the dropped packets from the end
user so long as they account for a small percentage
of the total.
TCP Windowing
One way to structure a communications protocol is
to have the receiver acknowledge every packet
received from a sender. Although this is the most
reliable method, it can add unnecessary overhead,
especially on fairly reliable connection media.
Windowing is a compromise that reduces overhead
by acknowledging packets only after a specified
number have been received.
The window size from one end station informs the
other side of the connection how much it can accept
at one time. With a window size of 1, each segment
must be acknowledged before another segment is
sent. This is the least efficient use of bandwidth. A
window size of 7 means that an acknowledgment
needs to be sent after the receipt of seven segments;
this allows better utilization of bandwidth. A windowing
example is shown in the figure.
to have the receiver acknowledge every packet
received from a sender. Although this is the most
reliable method, it can add unnecessary overhead,
especially on fairly reliable connection media.
Windowing is a compromise that reduces overhead
by acknowledging packets only after a specified
number have been received.
The window size from one end station informs the
other side of the connection how much it can accept
at one time. With a window size of 1, each segment
must be acknowledged before another segment is
sent. This is the least efficient use of bandwidth. A
window size of 7 means that an acknowledgment
needs to be sent after the receipt of seven segments;
this allows better utilization of bandwidth. A windowing
example is shown in the figure.
How TCP Connections Are Established
End stations exchange control bits called SYN (for
synchronize) and Initial Sequence Numbers (ISN)
to synchronize during connection establishment.
TCP/IP uses what is known as a three-way handshake
to establish connections.
To synchronize the connection, each side sends its
own initial sequence number and expects to
receive a confirmation in an acknowledgment
(ACK) from the other side. The following figure
shows an example.
synchronize) and Initial Sequence Numbers (ISN)
to synchronize during connection establishment.
TCP/IP uses what is known as a three-way handshake
to establish connections.
To synchronize the connection, each side sends its
own initial sequence number and expects to
receive a confirmation in an acknowledgment
(ACK) from the other side. The following figure
shows an example.
TCP/IP Datagrams
TCP/IP information is sent via datagrams. A single
message may be broken into a series of datagrams
that must be reassembled at their destination. Three
layers are associated with the TCP/IP protocol stack:
• Application layer: This layer specifies protocols
for e-mail, file transfer, remote login, and other
applications. Network management is also supported.
• Transport layer: This layer allows multiple
upper-layer applications to use the same data
stream. TCP and UDP protocols provide flow
control and reliability.
• Network layer: Several protocols operate at the
network layer, including IP, ICMP, ARP, and
RARP.
IP provides connectionless, best-effort routing of
datagrams.
TCP/IP hosts use Internet Control Message
Protocol (ICMP) to carry error and control messages
with IP datagrams. For example, a process
called ping allows one station to discover a host
on another network.
Address Resolution Protocol (ARP) allows communication
on a multiaccess medium such as
Ethernet by mapping known IP addresses to MAC
addresses.
message may be broken into a series of datagrams
that must be reassembled at their destination. Three
layers are associated with the TCP/IP protocol stack:
• Application layer: This layer specifies protocols
for e-mail, file transfer, remote login, and other
applications. Network management is also supported.
• Transport layer: This layer allows multiple
upper-layer applications to use the same data
stream. TCP and UDP protocols provide flow
control and reliability.
• Network layer: Several protocols operate at the
network layer, including IP, ICMP, ARP, and
RARP.
IP provides connectionless, best-effort routing of
datagrams.
TCP/IP hosts use Internet Control Message
Protocol (ICMP) to carry error and control messages
with IP datagrams. For example, a process
called ping allows one station to discover a host
on another network.
Address Resolution Protocol (ARP) allows communication
on a multiaccess medium such as
Ethernet by mapping known IP addresses to MAC
addresses.
What Problems Need to Be Solved?
TCP is a connection-oriented, reliable protocol that
breaks messages into segments and reassembles
them at the destination station (it also resends
packets not received at the destination). TCP also
provides virtual circuits between applications.
A connection-oriented protocol establishes and
maintains a connection during a transmission. The
protocol must establish the connection before sending
data. As soon as the data transfer is complete,
the session is torn down.
User Datagram Protocol (UDP) is an alternative
protocol to TCP that also operates at Layer 4. UDP
is considered an “unreliable,” connectionless protocol.
Although “unreliable” may have a negative
connotation, in cases where real-time information is
being exchanged (such as a voice conversation),
taking the time to set up a connection and resend
dropped packets can do more harm than good.
breaks messages into segments and reassembles
them at the destination station (it also resends
packets not received at the destination). TCP also
provides virtual circuits between applications.
A connection-oriented protocol establishes and
maintains a connection during a transmission. The
protocol must establish the connection before sending
data. As soon as the data transfer is complete,
the session is torn down.
User Datagram Protocol (UDP) is an alternative
protocol to TCP that also operates at Layer 4. UDP
is considered an “unreliable,” connectionless protocol.
Although “unreliable” may have a negative
connotation, in cases where real-time information is
being exchanged (such as a voice conversation),
taking the time to set up a connection and resend
dropped packets can do more harm than good.
Why Should I Care About TCP/IP?
TCP/IP is the best-known and most popular protocol
suite used today. Its ease of use and widespread
adoption are some of the best reasons for the
Internet explosion that is taking place.
Encompassed within the TCP/IP protocol is the
capability to offer reliable, connection-based packet
transfer (sometimes called synchronous) as well
as less reliable, connectionless transfers (also called
asynchronous).
suite used today. Its ease of use and widespread
adoption are some of the best reasons for the
Internet explosion that is taking place.
Encompassed within the TCP/IP protocol is the
capability to offer reliable, connection-based packet
transfer (sometimes called synchronous) as well
as less reliable, connectionless transfers (also called
asynchronous).
Domain Names and Relationship to IP Addresses
Because IP addresses are difficult to remember in their dotted-decimal notation,
a naming convention called domain names was established that’s more
natural for people to use. Domain names such as www.cisco.com are registered
and associated with a particular public IP address. The Domain Name
System (DNS) maps a readable name to an IP address. For example, when you
enter http://www.cisco.com into a browser, the PC uses the DNS protocol to
contact a DNS name server. The name server translates the name
http://www.cisco.com into the actual IP address for that host..
a naming convention called domain names was established that’s more
natural for people to use. Domain names such as www.cisco.com are registered
and associated with a particular public IP address. The Domain Name
System (DNS) maps a readable name to an IP address. For example, when you
enter http://www.cisco.com into a browser, the PC uses the DNS protocol to
contact a DNS name server. The name server translates the name
http://www.cisco.com into the actual IP address for that host..
Dynamically Allocated IP Addresses
A network administrator is responsible for assigning which devices receive
which IP addresses in a corporate network. The admin assigns an IP address to
a device in one of two ways: by configuring the device with a specific address
or by letting the device automatically learn its address from the network.
Dynamic Host Configuration Protocol (DHCP) is the protocol used for automatic
IP address assignment. Dynamic addressing saves considerable administrative
effort and conserves IP addressing space. It can be difficult to manually
administer IP addresses for every computer and device on a network. Most
networks use DHCP to automatically assign an available IP address to a device
when it connects to the network. Generally, devices that don’t move around
receive fixed addresses, known as static addressing. For example, servers,
routers, and switches usually receive static IP addresses. The rest use dynamic
addressing. For home networks you do not need a network administrator to
set up your address; instead, a home broadband router allocates IP addresses
via DHCP.
which IP addresses in a corporate network. The admin assigns an IP address to
a device in one of two ways: by configuring the device with a specific address
or by letting the device automatically learn its address from the network.
Dynamic Host Configuration Protocol (DHCP) is the protocol used for automatic
IP address assignment. Dynamic addressing saves considerable administrative
effort and conserves IP addressing space. It can be difficult to manually
administer IP addresses for every computer and device on a network. Most
networks use DHCP to automatically assign an available IP address to a device
when it connects to the network. Generally, devices that don’t move around
receive fixed addresses, known as static addressing. For example, servers,
routers, and switches usually receive static IP addresses. The rest use dynamic
addressing. For home networks you do not need a network administrator to
set up your address; instead, a home broadband router allocates IP addresses
via DHCP.
What Is an Address?
For computers to send and receive information to each other, they must have
some form of addressing so that each end device on the network knows what
information to read and what information to ignore. This capability is important
both for the computers that ultimately use the information and for the
devices that deliver information to end stations, such as switches and routers.
Every computer on a network has two addresses:
• MAC address: A manufacturer-allocated ID number (such as a global serial
number) that is permanent and unique to every network device on Earth.
MAC addresses are analogous to a social security number or other national
identification number. You have only one, it stays the same wherever you go,
and no two people (devices) have the same number. MAC address are formatted
using six pairs of hexadecimal numbers, such as 01-23-45-67-89-AB.
Hexadecimal or “hex” is a base 16 numbering scheme that uses the numbers
0 through 9 and the letters A through F to count from 0 to 15. This
might seem odd, but it provides an easy translation from binary (which uses
only 1s and 0s), which is the language of all computers.
• IP address: This address is what matters most to basic networking. Unlike a
MAC address, the IP address of any device is temporary and can be
changed. It is often assigned by the network itself and is analogous to your
street address. It only needs to be unique within a network. Someone else’s
network might use the same IP address, much like another town might have
the same street (for example, 101 Main Street). Every device on an IP network
is given an IP address, which looks like this: 192.168.1.100.
The format of this address is called dotted-decimal notation. The period separators
are pronounced “dot,” as in one ninety two dot one sixty eight dot....”
Because of some rules with binary, the largest number in each section is 255.
In addition to breaking up the number, the dots that appear in IP addresses
allow us to break the address into parts that represent networks and hosts. In
this case, the “network” portion refers to a company, university, government
agency, or your private network. The hosts would be the addresses of all the
computers on the individual network. If you think of the network portion of
the address as a street, the hosts would be all the houses on that street. If you
could see the IP addresses of everyone who is on the same network segment as
you, you would notice that the network portion of the address is the same for
all computers, and the host portion changes from computer to computer. An
example will probably help. Think of an IP address as being like your home
address for the post office: state.city.street.house-number.
Each number in the IP address provides a more and more specific location so
that the Internet can find your computer among millions of other computers.
The Internet is not organized geographically like the postal system, though.
The components of the address (intentionally oversimplified) are majornetwork.
minor-network.local-network.device.
some form of addressing so that each end device on the network knows what
information to read and what information to ignore. This capability is important
both for the computers that ultimately use the information and for the
devices that deliver information to end stations, such as switches and routers.
Every computer on a network has two addresses:
• MAC address: A manufacturer-allocated ID number (such as a global serial
number) that is permanent and unique to every network device on Earth.
MAC addresses are analogous to a social security number or other national
identification number. You have only one, it stays the same wherever you go,
and no two people (devices) have the same number. MAC address are formatted
using six pairs of hexadecimal numbers, such as 01-23-45-67-89-AB.
Hexadecimal or “hex” is a base 16 numbering scheme that uses the numbers
0 through 9 and the letters A through F to count from 0 to 15. This
might seem odd, but it provides an easy translation from binary (which uses
only 1s and 0s), which is the language of all computers.
• IP address: This address is what matters most to basic networking. Unlike a
MAC address, the IP address of any device is temporary and can be
changed. It is often assigned by the network itself and is analogous to your
street address. It only needs to be unique within a network. Someone else’s
network might use the same IP address, much like another town might have
the same street (for example, 101 Main Street). Every device on an IP network
is given an IP address, which looks like this: 192.168.1.100.
The format of this address is called dotted-decimal notation. The period separators
are pronounced “dot,” as in one ninety two dot one sixty eight dot....”
Because of some rules with binary, the largest number in each section is 255.
In addition to breaking up the number, the dots that appear in IP addresses
allow us to break the address into parts that represent networks and hosts. In
this case, the “network” portion refers to a company, university, government
agency, or your private network. The hosts would be the addresses of all the
computers on the individual network. If you think of the network portion of
the address as a street, the hosts would be all the houses on that street. If you
could see the IP addresses of everyone who is on the same network segment as
you, you would notice that the network portion of the address is the same for
all computers, and the host portion changes from computer to computer. An
example will probably help. Think of an IP address as being like your home
address for the post office: state.city.street.house-number.
Each number in the IP address provides a more and more specific location so
that the Internet can find your computer among millions of other computers.
The Internet is not organized geographically like the postal system, though.
The components of the address (intentionally oversimplified) are majornetwork.
minor-network.local-network.device.
Computers Speaking the Same Language
The Internet protocols comprise the most popular, nonproprietary data-networking
protocol suite in the world. The Internet protocols are communication protocols
used by electronic devices to talk to each other. Initially, computers were the
primary clients of IP protocols, but other types of electronic devices can connect
to IP networks, including printers, cellular phones, and MP3 players.
Today, even common devices such as vending machines, dishwashers, and cars
are being connected to IP networks.
The two best-known Internet protocols are Transmission Control Protocol
(TCP) and Internet Protocol (IP). The Defense Advanced Research Projects
Agency (DARPA) developed the Internet protocols in the mid-1970s. DARPA
funded Stanford University and Bolt, Beranek, and Newman (BBN) to develop
a set of protocols that would allow different types of computers at various
research locations to communicate over a common packet-switched network.
The result of this research produced the Internet protocol suite, which was
later distributed for free with the Berkeley Software Distribution (BSD) UNIX
operating system.
From there, IP became the primary networking protocol, serving as the basis
for the World Wide Web (WWW) and the Internet in general. Internet protocols
are discussed and adopted in the public domain. Technical bulletins called
Requests for Comments (RFC) documents proposed protocols and practices.
These documents are reviewed, edited, published, and analyzed, and then are
accepted by the Internet community (this process takes years).
The Internet protocol suite also comprises application-based protocols, including
definitions for the following:
• Electronic mail (Simple Mail Transfer Protocol [SMTP])
• Terminal emulation (Telnet)
• File transfer (File Transfer Protocol [FTP])
• HTTP
IP is considered a Layer 3 protocol according to the OSI model, and TCP is a
Layer 4 protocol.
protocol suite in the world. The Internet protocols are communication protocols
used by electronic devices to talk to each other. Initially, computers were the
primary clients of IP protocols, but other types of electronic devices can connect
to IP networks, including printers, cellular phones, and MP3 players.
Today, even common devices such as vending machines, dishwashers, and cars
are being connected to IP networks.
The two best-known Internet protocols are Transmission Control Protocol
(TCP) and Internet Protocol (IP). The Defense Advanced Research Projects
Agency (DARPA) developed the Internet protocols in the mid-1970s. DARPA
funded Stanford University and Bolt, Beranek, and Newman (BBN) to develop
a set of protocols that would allow different types of computers at various
research locations to communicate over a common packet-switched network.
The result of this research produced the Internet protocol suite, which was
later distributed for free with the Berkeley Software Distribution (BSD) UNIX
operating system.
From there, IP became the primary networking protocol, serving as the basis
for the World Wide Web (WWW) and the Internet in general. Internet protocols
are discussed and adopted in the public domain. Technical bulletins called
Requests for Comments (RFC) documents proposed protocols and practices.
These documents are reviewed, edited, published, and analyzed, and then are
accepted by the Internet community (this process takes years).
The Internet protocol suite also comprises application-based protocols, including
definitions for the following:
• Electronic mail (Simple Mail Transfer Protocol [SMTP])
• Terminal emulation (Telnet)
• File transfer (File Transfer Protocol [FTP])
• HTTP
IP is considered a Layer 3 protocol according to the OSI model, and TCP is a
Layer 4 protocol.
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