UNIT 4
NETWORK LAYER AND TRANSPORT LAYER
Repeaters, Bridges, gateways and routers
Repeaters
A repeater is a device that operates only in the physical layer. Signals that carry information
within a network can travel a fixed distance before attenuation endangers the integrity of the
data. A repeater receives a signal and, before it becomes too weak or corrupted, regenerates
the original bit pattern. The repeater then sends the refreshed signal. A repeater can extend
the physical length of a LAN, as shown in Figure 15.2.
A repeater does not actually connect two LANs; it connects two segments of the same LAN.
The segments connected are still part of one single LAN. A repeater is not a device that can
connect two LANs of different protocols.
A repeater connects segments of a LAN.
A repeater can overcome the 10Base5 Ethernet length restriction. In this standard, the length
of the cable is limited to 500 m. To extend this length, we divide the cable into segments and
install repeaters between segments. Note that the whole network is still considered one LAN,
but the portions of the network separated by repeaters are called segments. The repeater acts
as a two-port node, but operates only in the physical layer. When it receives a frame from any
of the ports, it regenerates and forwards it to the other port.
A repeater forwards every frame; it has no filtering capability.
An amplifier cannot discriminate between the intended signal and noise; it amplifies equally
everything fed into it. A repeater does not amplify the signal; it regenerates the signal. When
it receives a weakened or corrupted signal, it creates a copy, bit for bit, at the original
strength.
A repeater is a regenerator, not an amplifier.
A repeater must be placed so that a signal reaches it before any noise changes the meaning of
any of its bits. A little noise can alter the precision of a bit's voltage without destroying its
identity (see Figure 15.3). If the corrupted bit travels much farther, however, accumulated
noise can change its meaning completely. At that point, the original voltage is not
recoverable, and the error needs to be corrected. A repeater placed on the line before the
legibility of the signal becomes lost can still read the signal well enough to determine the
intended voltages and replicate them in their original form.
,Bridges
A bridge operates in both the physical and the data link layer. As a physical layer device, it
regenerates the signal it receives. As a data link layer device, the bridge can check the
physical (MAC) addresses (source and destination) contained in the frame.
Filtering
A bridge has filtering capability. It can check the destination address of a frame and decide if
the frame should be forwarded or dropped. If the frame is to be forwarded, the decision must
specify the port. A bridge has a table that maps addresses to ports. A bridge does not change
the physical addresses contained in the frame.
A bridge does not change the physical (MAC) addresses in a frame.
,Transparent Bridges
A transparent bridge is a bridge in which the stations are completely unaware of the bridge's
existence. If a bridge is added or deleted from the system, reconfiguration of the stations is
unnecessary.
Routers
A router is a three-layer device that routes packets based on their logical addresses (host-to-
host addressing). A router normally connects LANs and WANs in the Internet and has a
routing table that is used for making decisions about the route. The routing tables are
normally dynamic and are updated using routing protocols. Figure 15.11 shows a part of the
Internet that uses routers to connect LANs and WANs.
Gateway
A gateway is normally a computer that operates in all five layers of the Internet or seven
layers of OSI model. A gateway takes an application message, reads it, and interprets it. This
means that it can be used as a connecting device between two internetworks that use different
models. For example, a network designed to use the OSI model can be connected to another
network using the Internet model. The gateway connecting the two systems can take a frame
as it arrives from the first system, move it up to the OSI application layer, and remove the
message. Gateways can provide security. The gateway is used to filter unwanted application-
layer messages.
, UNIT 4
LOGICAL ADDRESSING-IPV4 AND IPV6 ADDRESSING, INTERNET PROTOCOL-
IPV4 AND IPV6
IPv4 ADDRESSES
An IPv4 address is a 32-bit address that uniquely and universally defines the connection of a
device (for example, a computer or a router) to the Internet.
IPv4 addresses are unique. They are unique in the sense that each address defines one, and only
one, connection to the Internet. Two devices on the Internet can never have the same address at
the same time. The IPv4 addresses are unique and universal.
Address Space
A protocol such as IPv4 that defines addresses has an address space. An address space is the
total number of addresses used by the protocol. If a protocol uses N bits to define an address, the
address space is 2N because each bit can have two different values (0 or 1) and N bits can have 2N
values.
IPv4 uses 32-bit addresses, which means that the address space is 232 or 4,294,967,296 (more
than 4 billion). This means that, theoretically, if there were no restrictions, more than 4 billion
devices could be connected to the Internet.
The address space of IPv4 is 232 or 4,294,967,296.
Notations
There are two prevalent notations to show an IPv4 address: binary notation and dotted-decimal
notation.
Binary Notation
In binary notation, the IPv4 address is displayed as 32 bits. Each octet is often referred to as a
byte. An IPv4 address is referred to as a 32-bit address or a 4-byte address. The following is an
example of an IPv4 address in binary notation:
01110101 10010101 00011101 00000010
Dotted-Decimal Notation
To make the IPv4 address more compact and easier to read, Internet addresses are usually written
in decimal form with a decimal point (dot) separating the bytes. The following is the dotted-
decimal notation of the above address:
117.149.29.2
Figure 19.1 shows an IPv4 address in both binary and dotted-decimal notation. Note that because
each byte (octet) is 8 bits, each number in dotted-decimal notation is a value ranging from 0 to
255.
NETWORK LAYER AND TRANSPORT LAYER
Repeaters, Bridges, gateways and routers
Repeaters
A repeater is a device that operates only in the physical layer. Signals that carry information
within a network can travel a fixed distance before attenuation endangers the integrity of the
data. A repeater receives a signal and, before it becomes too weak or corrupted, regenerates
the original bit pattern. The repeater then sends the refreshed signal. A repeater can extend
the physical length of a LAN, as shown in Figure 15.2.
A repeater does not actually connect two LANs; it connects two segments of the same LAN.
The segments connected are still part of one single LAN. A repeater is not a device that can
connect two LANs of different protocols.
A repeater connects segments of a LAN.
A repeater can overcome the 10Base5 Ethernet length restriction. In this standard, the length
of the cable is limited to 500 m. To extend this length, we divide the cable into segments and
install repeaters between segments. Note that the whole network is still considered one LAN,
but the portions of the network separated by repeaters are called segments. The repeater acts
as a two-port node, but operates only in the physical layer. When it receives a frame from any
of the ports, it regenerates and forwards it to the other port.
A repeater forwards every frame; it has no filtering capability.
An amplifier cannot discriminate between the intended signal and noise; it amplifies equally
everything fed into it. A repeater does not amplify the signal; it regenerates the signal. When
it receives a weakened or corrupted signal, it creates a copy, bit for bit, at the original
strength.
A repeater is a regenerator, not an amplifier.
A repeater must be placed so that a signal reaches it before any noise changes the meaning of
any of its bits. A little noise can alter the precision of a bit's voltage without destroying its
identity (see Figure 15.3). If the corrupted bit travels much farther, however, accumulated
noise can change its meaning completely. At that point, the original voltage is not
recoverable, and the error needs to be corrected. A repeater placed on the line before the
legibility of the signal becomes lost can still read the signal well enough to determine the
intended voltages and replicate them in their original form.
,Bridges
A bridge operates in both the physical and the data link layer. As a physical layer device, it
regenerates the signal it receives. As a data link layer device, the bridge can check the
physical (MAC) addresses (source and destination) contained in the frame.
Filtering
A bridge has filtering capability. It can check the destination address of a frame and decide if
the frame should be forwarded or dropped. If the frame is to be forwarded, the decision must
specify the port. A bridge has a table that maps addresses to ports. A bridge does not change
the physical addresses contained in the frame.
A bridge does not change the physical (MAC) addresses in a frame.
,Transparent Bridges
A transparent bridge is a bridge in which the stations are completely unaware of the bridge's
existence. If a bridge is added or deleted from the system, reconfiguration of the stations is
unnecessary.
Routers
A router is a three-layer device that routes packets based on their logical addresses (host-to-
host addressing). A router normally connects LANs and WANs in the Internet and has a
routing table that is used for making decisions about the route. The routing tables are
normally dynamic and are updated using routing protocols. Figure 15.11 shows a part of the
Internet that uses routers to connect LANs and WANs.
Gateway
A gateway is normally a computer that operates in all five layers of the Internet or seven
layers of OSI model. A gateway takes an application message, reads it, and interprets it. This
means that it can be used as a connecting device between two internetworks that use different
models. For example, a network designed to use the OSI model can be connected to another
network using the Internet model. The gateway connecting the two systems can take a frame
as it arrives from the first system, move it up to the OSI application layer, and remove the
message. Gateways can provide security. The gateway is used to filter unwanted application-
layer messages.
, UNIT 4
LOGICAL ADDRESSING-IPV4 AND IPV6 ADDRESSING, INTERNET PROTOCOL-
IPV4 AND IPV6
IPv4 ADDRESSES
An IPv4 address is a 32-bit address that uniquely and universally defines the connection of a
device (for example, a computer or a router) to the Internet.
IPv4 addresses are unique. They are unique in the sense that each address defines one, and only
one, connection to the Internet. Two devices on the Internet can never have the same address at
the same time. The IPv4 addresses are unique and universal.
Address Space
A protocol such as IPv4 that defines addresses has an address space. An address space is the
total number of addresses used by the protocol. If a protocol uses N bits to define an address, the
address space is 2N because each bit can have two different values (0 or 1) and N bits can have 2N
values.
IPv4 uses 32-bit addresses, which means that the address space is 232 or 4,294,967,296 (more
than 4 billion). This means that, theoretically, if there were no restrictions, more than 4 billion
devices could be connected to the Internet.
The address space of IPv4 is 232 or 4,294,967,296.
Notations
There are two prevalent notations to show an IPv4 address: binary notation and dotted-decimal
notation.
Binary Notation
In binary notation, the IPv4 address is displayed as 32 bits. Each octet is often referred to as a
byte. An IPv4 address is referred to as a 32-bit address or a 4-byte address. The following is an
example of an IPv4 address in binary notation:
01110101 10010101 00011101 00000010
Dotted-Decimal Notation
To make the IPv4 address more compact and easier to read, Internet addresses are usually written
in decimal form with a decimal point (dot) separating the bytes. The following is the dotted-
decimal notation of the above address:
117.149.29.2
Figure 19.1 shows an IPv4 address in both binary and dotted-decimal notation. Note that because
each byte (octet) is 8 bits, each number in dotted-decimal notation is a value ranging from 0 to
255.
