CS6250 Computer Networks Exam Questions with
Answers Graded A+ 2025/2026
Describe each layer of the OSI model. - Application layer: Service,
Interface, Protocol. Ex: Turn on your smartphone and look at the list of
apps. (HTTP, SMTP, FTP, DNS)
Presentation layer: Plays intermediate role of formatting the information
received from the layer below and delivering it to the application layer. Ex:
converting big endian to little endian.
Session layer: Responsible for the mechanism that manages the different
transport streams that belong to the same session between end-user and
application process. Ex: teleconference app, it is responsible for tying
together audio and video streaming.
Transport layer: Responsible for the end-to-end communication between
end hosts. 2 transport protocols, TCP and UDP. TCP includes a
connection-oriented service to the applications that are running on the layer
above, guaranteed delivery of the application-layer messages, flow control,
and congestion control mechanism. UDP provides a connectionless, best-
effort service to the applications that are running in the layer above without
reliability, flow, or congestion control. In this layer the packet is called a
segment.
Network layer: This layer is responsible for moving the packet of
information, called a datagram, from one host to another. The network layer
is responsible for delivering the datagram to the Transport layer on the
destination host. In this layer there are the IP Protocol and the routing
tables.
Data Link layer: Packets are referred to as frames. Examples include:
ethernet, ppp, wifi. Responsible for moving the frames from one node (host
or router) to the next node. Services offered by the data link layer protocol
include reliable delivery (transmission of the data from one transmitting
node, across one link, to the receiving node.
Physical layer: This layer is the actual hardware responsible to transfer bits
within a frame between two nodes c
Provide examples of popular protocols at each layer of the five-layered
Internet model. - Application: NFS, DNS, SNMP, ftp, rcp, telnet, HTTP
Transport: TCP, UDP
Internet: IP, ARP, ICMP
Data Link: PPP, IEEE 802.2, Ethernet
Physical Network: Token Ring, RS-232
,What is encapsulation, and how is it used in a layered model? -
Encapsulation is when data (called a header) is appended to the packet
through each layer to signify its on the correct path to the destination host.
What is the end-to-end (e2e) principle? - A design choice that shaped the
current internet architecture. It states the network core should be simple
and minimal, while the end systems should carry the intelligence. Network
functions should be simple and essential commonly used functions so any
host can utilize the service and higher form functions should be built into
the application itself. Lower level layers should be independent and free to
perform only their designed function and the higher-level layers deal with
the more intricate functions that deal with the specific application.
What are the examples of a violation of e2e principle? - Violations include
firewalls and traffic filters. Firewalls violate because they are intermediate
devices that are operated between two end hosts and they can drop the
end host communications. Network Address Translation (NAT) boxes are
also a violation because it uses the single public IP address and distributes
a new IP scheme to the hosts connected to it to route data through re-
writing the header info to route to the correct destination host. NAT boxes
are a violation because they are not globally addressable or routable.
What is the EvoArch model? - An hourglass shaped model of the Internet
where the outer bands are more frequently modified or replaced and the
further in you go the harder it is for that layer to be altered or modified.
Explain a round in the EvoArch model. - EvoArch is a discrete-time model
that is executed over rounds. At each round, we perform the following
steps: A) We introduce new nodes, and we place them randomly at layers.
B) We examine all layers, from the top to the bottom, and we perform the
following tasks: 1) We connect the new nodes that we may have just
introduced to that layer, by choosing substrates based on the generality
probabilities of the layer below s(l−1), and by choosing products for them
based on the generality probability of the current layer s(l). 2) We update
the value of each node at each layer l, given that we may have new nodes
added to the same layer l. 3) We examine all nodes, in order of decreasing
value in that layer, and remove the nodes that should die. C) Finally, we
stop the execution of the model when the network reaches a given number
of nodes.
, What are the ramifications of the hourglass shape of the internet? - A.
Many technologies that were not originally designed for the internet have
been modified so that they have versions that can communicate over the
internet (such as Radio over IP).
B. It has been a difficult and slow process to transition to IPv6, despite the
shortage of public IPv4 addresses.
Repeaters, hubs, bridges, routers operate on which layers? - Repeaters
and Hubs work over L1 (Physical Layer)
Bridges and Layer 2-Switches work over L2 (Data link layer)
Routers and Layer 3-Switches work over L3 (Network layer)
What is a bridge, and how does it "learn"? - A bridge is a device with
multiple inputs/outputs. A bridge transfers frames from an input to one (or
multiple) outputs. Though it doesn't need to forward all the frames it
receives.
A learning bridge learns, populates and maintains a forwarding table. The
bridge consults that table so that it only forwards frames on specific ports,
rather than over all ports. So how does the bridge learn? When the bridge
receives any frame this is a "learning opportunity" to know which hosts are
reachable through which ports. This is because the bridge can view the
port over which a frame arrives and the source host.
What is a distributed algorithm? - A distributed algorithm is an algorithm
designed to run on computer hardware constructed from interconnected
processors. Distributed algorithms are used in many varied application
areas of distributed computing, such as telecommunications, scientific
computing, distributed information processing, and real-time process
control.
https://en.wikipedia.org/wiki/Distributed_algorithm
Explain the Spanning Tree Algorithm. - The algorithm runs in "rounds" and
at every round each node sends to each neighbor node a configuration
message with three fields: a) the sending node's ID, b) the ID of the roots
as perceived by the sending node, and c) the number of hops between that
(perceived) root and the sending node.
Answers Graded A+ 2025/2026
Describe each layer of the OSI model. - Application layer: Service,
Interface, Protocol. Ex: Turn on your smartphone and look at the list of
apps. (HTTP, SMTP, FTP, DNS)
Presentation layer: Plays intermediate role of formatting the information
received from the layer below and delivering it to the application layer. Ex:
converting big endian to little endian.
Session layer: Responsible for the mechanism that manages the different
transport streams that belong to the same session between end-user and
application process. Ex: teleconference app, it is responsible for tying
together audio and video streaming.
Transport layer: Responsible for the end-to-end communication between
end hosts. 2 transport protocols, TCP and UDP. TCP includes a
connection-oriented service to the applications that are running on the layer
above, guaranteed delivery of the application-layer messages, flow control,
and congestion control mechanism. UDP provides a connectionless, best-
effort service to the applications that are running in the layer above without
reliability, flow, or congestion control. In this layer the packet is called a
segment.
Network layer: This layer is responsible for moving the packet of
information, called a datagram, from one host to another. The network layer
is responsible for delivering the datagram to the Transport layer on the
destination host. In this layer there are the IP Protocol and the routing
tables.
Data Link layer: Packets are referred to as frames. Examples include:
ethernet, ppp, wifi. Responsible for moving the frames from one node (host
or router) to the next node. Services offered by the data link layer protocol
include reliable delivery (transmission of the data from one transmitting
node, across one link, to the receiving node.
Physical layer: This layer is the actual hardware responsible to transfer bits
within a frame between two nodes c
Provide examples of popular protocols at each layer of the five-layered
Internet model. - Application: NFS, DNS, SNMP, ftp, rcp, telnet, HTTP
Transport: TCP, UDP
Internet: IP, ARP, ICMP
Data Link: PPP, IEEE 802.2, Ethernet
Physical Network: Token Ring, RS-232
,What is encapsulation, and how is it used in a layered model? -
Encapsulation is when data (called a header) is appended to the packet
through each layer to signify its on the correct path to the destination host.
What is the end-to-end (e2e) principle? - A design choice that shaped the
current internet architecture. It states the network core should be simple
and minimal, while the end systems should carry the intelligence. Network
functions should be simple and essential commonly used functions so any
host can utilize the service and higher form functions should be built into
the application itself. Lower level layers should be independent and free to
perform only their designed function and the higher-level layers deal with
the more intricate functions that deal with the specific application.
What are the examples of a violation of e2e principle? - Violations include
firewalls and traffic filters. Firewalls violate because they are intermediate
devices that are operated between two end hosts and they can drop the
end host communications. Network Address Translation (NAT) boxes are
also a violation because it uses the single public IP address and distributes
a new IP scheme to the hosts connected to it to route data through re-
writing the header info to route to the correct destination host. NAT boxes
are a violation because they are not globally addressable or routable.
What is the EvoArch model? - An hourglass shaped model of the Internet
where the outer bands are more frequently modified or replaced and the
further in you go the harder it is for that layer to be altered or modified.
Explain a round in the EvoArch model. - EvoArch is a discrete-time model
that is executed over rounds. At each round, we perform the following
steps: A) We introduce new nodes, and we place them randomly at layers.
B) We examine all layers, from the top to the bottom, and we perform the
following tasks: 1) We connect the new nodes that we may have just
introduced to that layer, by choosing substrates based on the generality
probabilities of the layer below s(l−1), and by choosing products for them
based on the generality probability of the current layer s(l). 2) We update
the value of each node at each layer l, given that we may have new nodes
added to the same layer l. 3) We examine all nodes, in order of decreasing
value in that layer, and remove the nodes that should die. C) Finally, we
stop the execution of the model when the network reaches a given number
of nodes.
, What are the ramifications of the hourglass shape of the internet? - A.
Many technologies that were not originally designed for the internet have
been modified so that they have versions that can communicate over the
internet (such as Radio over IP).
B. It has been a difficult and slow process to transition to IPv6, despite the
shortage of public IPv4 addresses.
Repeaters, hubs, bridges, routers operate on which layers? - Repeaters
and Hubs work over L1 (Physical Layer)
Bridges and Layer 2-Switches work over L2 (Data link layer)
Routers and Layer 3-Switches work over L3 (Network layer)
What is a bridge, and how does it "learn"? - A bridge is a device with
multiple inputs/outputs. A bridge transfers frames from an input to one (or
multiple) outputs. Though it doesn't need to forward all the frames it
receives.
A learning bridge learns, populates and maintains a forwarding table. The
bridge consults that table so that it only forwards frames on specific ports,
rather than over all ports. So how does the bridge learn? When the bridge
receives any frame this is a "learning opportunity" to know which hosts are
reachable through which ports. This is because the bridge can view the
port over which a frame arrives and the source host.
What is a distributed algorithm? - A distributed algorithm is an algorithm
designed to run on computer hardware constructed from interconnected
processors. Distributed algorithms are used in many varied application
areas of distributed computing, such as telecommunications, scientific
computing, distributed information processing, and real-time process
control.
https://en.wikipedia.org/wiki/Distributed_algorithm
Explain the Spanning Tree Algorithm. - The algorithm runs in "rounds" and
at every round each node sends to each neighbor node a configuration
message with three fields: a) the sending node's ID, b) the ID of the roots
as perceived by the sending node, and c) the number of hops between that
(perceived) root and the sending node.
