Multiplexer takes several inputs and gives a single output, while demultiplexer takes a single input and gives several outputs. They both follow combinational logic and work on different operational principles - multiplexer is many-to-one, while demultiplexer is one-to-many. Multiplexing and demultiplexing at the transport layer allow data from multiple applications to be transmitted simultaneously over a network from source to destination, where it is directed to the appropriate application.

1. Multiplexer
Multiplexer is a data selector which takes several inputs and
gives a single output.
In multiplexer we have 2n input lines and 1 output lines where n is
the number of selection lines.

3. Demultiplexer
• Demultiplexer is a data distributor which takes a single input
and gives several outputs.
• In demultiplexer we have 1 input and 2n output lines where n is
the selection line.

5. Difference between of Multiplexer and
Demultiplexer
Multiplexer Demultiplexer
Multiplexer processes the digital information from various
sources into a single source.
Demultiplexer receives digital information from a single
source and converts it into several sources
It is known as Data Selector It is known as Data Distributor
Multiplexer is a digital switch Demultiplexer is a digital circuit
It follows combinational logic type It also follows combinational logic type
It has n data input It has single data input
It has a single data output It has n data outputs
It works on many to one operational principle It works on one to many operational principle
In time division Multiplexing, multiplexer is used at the
transmitter end
In time division Multiplexing, demultiplexer is used at the
receiver end

6. Transport Layer

7. Multiplexing
• Multiplexing is the process of collecting the data from multiple
application processes of the sender, enveloping that data with headers
and sending them as a whole to the intended receiver.
• In Multiplexing at the Transport Layer, the data is collected from
various application processes. These segments contain the source port
number, destination port number, header files, and data.
• These segments are passed to the Network Layer which adds the
source and destination IP address to get the datagram.

8. Demultiplexing
• Delivering the received segments at the receiver side to the correct app
layer processes is called demultiplexing.
• The destination host receives the IP datagrams; each datagram has a
source IP address and a destination IP address.
• Each datagram carries 1 transport layer segment.
• Each segment has the source and destination port number.
• The destination host uses the IP addresses and port numbers to direct
the segment to the appropriate socket.

9. • Multiplexing and demultiplexing are just concepts that describe the
process of the transmission of data generated by different applications
simultaneously.
• When the data arrives at the Transport layer, each data segment is
independently processed and sent to its appropriate application in the
destination machine.

10. • The main objective of multiplexing and demultiplexing is to allow us
to use a multitude of applications simultaneously.
• The above figure shows that the source computer is using Google,
Outlook, and Chat applications at the same time.
• All the data is forwarded to a destination computer.
• Each application has a segment put on a wire to be transmitted. It
signifies that all applications are running simultaneously.
• Without multiplexing/demultiplexing exists, a user can use only one
application at a time because only the segments of that application are
put on the wire and transmitted.

12. • In the above figure, the Application layer has generated data, and then
passed it down to the Transport layer to be segmented.
• After segmenting the data, port numbers are given to each segment to
be ready for transmission.
• Then the segments are put on a wire to travel across the network to the
destination. This process is called "multiplexing".
• When the transmitted segments reach the Transport layer of the
destination, they are automatically sent up to their appropriate
applications. This process is called "demultiplexing".

13. Network Layer

14. Introduction
• Every computer in a network has an IP address by which it can be
uniquely identified and addressed.
• An IP address is Layer-3 (Network Layer) logical address. This
address may change every time a computer restarts.
• A computer can have one IP at one instance of time and another IP at
some different time.

15. Address Resolution Protocol(ARP)
• While communicating, a host needs Layer-2 (MAC) address of the
destination machine which belongs to the same broadcast domain or
network.
• A MAC address is physically burnt into the Network Interface Card
(NIC) of a machine and it never changes.
• On the other hand, IP address on the public domain is rarely changed.
If the NIC is changed in case of some fault, the MAC address also
changes.
• This way, for Layer-2 communication to take place, a mapping
between the two is required.

17. • To know the MAC address of remote host on a broadcast domain, a computer
wishing to initiate communication sends out an ARP broadcast message asking,
“Who has this IP address?” Because it is a broadcast, all hosts on the network
segment (broadcast domain) receive this packet and process it.
• ARP packet contains the IP address of destination host, the sending host wishes to
talk to. When a host receives an ARP packet destined to it, it replies back with its
own MAC address.
• Once the host gets destination MAC address, it can communicate with remote host
using Layer-2 link protocol. This MAC to IP mapping is saved into ARP cache of
both sending and receiving hosts. Next time, if they require to communicate, they
can directly refer to their respective ARP cache.
• Reverse ARP is a mechanism where host knows the MAC address of remote host
but requires to know IP address to communicate.

18. Internet Control Message Protocol (ICMP)
• ICMP is network diagnostic and error reporting protocol. ICMP belongs to
IP protocol suite and uses IP as carrier protocol. After constructing ICMP
packet, it is encapsulated in IP packet. Because IP itself is a best-effort non-
reliable protocol, so is ICMP.
• Any feedback about network is sent back to the originating host. If some
error in the network occurs, it is reported by means of ICMP. ICMP contains
dozens of diagnostic and error reporting messages.
• ICMP-echo and ICMP-echo-reply are the most commonly used ICMP
messages to check the reachability of end-to-end hosts. When a host
receives an ICMP-echo request, it is bound to send back an ICMP-echo-
reply. If there is any problem in the transit network, the ICMP will report
that problem.

19. Internet Protocol Version 4 (IPv4)
• IPv4 is 32-bit addressing scheme used as TCP/IP host addressing mechanism. IP addressing
enables every host on the TCP/IP network to be uniquely identifiable.
• IPv4 provides hierarchical addressing scheme which enables it to divide the network into sub-
networks, each with well-defined number of hosts. IP addresses are divided into many categories:
• Class A - it uses first octet for network addresses and last three octets for host addressing
• Class B - it uses first two octets for network addresses and last two for host addressing
• Class C - it uses first three octets for network addresses and last one for host addressing
• Class D - it provides flat IP addressing scheme in contrast to hierarchical structure for above three.
• Class E - It is used as experimental.
• IPv4 also has well-defined address spaces to be used as private addresses (not routable on internet),
and public addresses (provided by ISPs and are routable on internet).
• Though IP is not reliable one; it provides ‘Best-Effort-Delivery’ mechanism.

20. Internet Protocol Version 6 (IPv6)
• Exhaustion of IPv4 addresses gave birth to a next generation Internet Protocol version 6. IPv6 addresses its
nodes with 128-bit wide address providing plenty of address space for future to be used on entire planet or
beyond.
• IPv6 has introduced Anycast addressing but has removed the concept of broadcasting. IPv6 enables devices to
self-acquire an IPv6 address and communicate within that subnet. This auto-configuration removes the
dependability of Dynamic Host Configuration Protocol (DHCP) servers. This way, even if the DHCP server
on that subnet is down, the hosts can communicate with each other.
• IPv6 provides new feature of IPv6 mobility. Mobile IPv6 equipped machines can roam around without the
need of changing their IP addresses.
• IPv6 is still in transition phase and is expected to replace IPv4 completely in coming years. At present, there
are few networks which are running on IPv6. There are some transition mechanisms available for IPv6
enabled networks to speak and roam around different networks easily on IPv4. These are:
• Dual stack implementation
• Tunneling
• NAT-PT