COMPUTER NETWORKING EXAM 2025
QUESTIONS AND ANSWERS
R1. Let's review some of the terminology used in this textbook. Recall that the name of a
transport-layer packet is segment and that the name of a link-layer packet is frame. What is the
name of a network-layer packet? Recall that both routers and link-layer switches are called
packet switches. What is the fundamental difference between a router and link-layer switch?
Recall that we use the term routers for both datagram networks and VC networks. - ANS A
network-layer packet is a datagram. A router forwards a packet based on the packet's IP (layer
3) address. A link-layer switch forwards a packet based on the packet's MAC (layer 2) address.
R2. What are the two most important network-layer functions in a datagram network? What
are the three most important network-layer functions in a virtual-circuit network? - ANS The
two functions of datagram-based network layer are path determination and switching. The
additional function of a vc-based network is call setup.
R3. What is the difference between routing and forwarding? - ANS Forwarding is about
moving a packet from a router's input link to the appropriate output link. Routing is about
determining the end-to-routes between sources and destinations.
R4. Do the routers in both datagram networks and virtual-circuit network use forwarding
tables? If so, describe the forwarding tables for both classes of networks. - ANS Yes, both use
forwarding tables. For descriptions of the tables, see Section 4.2.
R5. Describe some hypothetical services that the network layer can provide to a single packet.
Do the same for a flow of packets. Are any of can provide to a single packet. Do the same for a
flow of packets. Are any of your hypothetical service provided by the Internet's network layer?
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 1
,Are any provided by ATM's CBR service model? Are any provided by ATM's ABR service model?
- ANS ...
R6. List some applications that would benefit from ATM's CBR service model. - ANS Single
packet: guaranteed delivery; guaranteed delivery with guaranteed maximum delay. Flow of
packets: in-order packet delivery; guaranteed minimal bandwidth; guaranteed maximum jitter.
None of these services is provided by the Internet's network layer. ATM's CBR service provides
both guaranteed delivery and timing. ABR does not provide any of these services.
R7. Discuss why each input port in a high-speed router stores shadow copy of the forwarding
table. - ANS With the shadow copy, the forwarding decision is made locally, at each input
port, without invoking the centralized routing processor. Such decentralized forwarding avoids
creating a forwarding processing bottleneck at a single point within the router.
R8. Three types of switching fabrics are discussed in Section 4.3. List and briefly describe each
type. Which, if any, can send multiple packets across the fabric in parallel. - ANS Switching
via memory; switching via a bus; switching via an interconnection network
R9. Describe how packet loss can occur at input ports. Describe how packet loss at input ports
can be eliminated (without using infinite buffers). - ANS Packet loss occurs if queue size at
the input port grows large because of slow switching fabric speed and thus exhausting router's
buffer space. It can be eliminated if the switching fabric speed is at least n times as fast as the
input line speed, where n is the number of input ports.
R10. Describe how packet loss can occur at input ports. Describe how packet loss at input ports
can be eliminated (without using infinite buffers). - ANS Packet loss can occur if the queue
size at the output port grows large because of slow outgoing line-speed.
R11. What is HOL blocking? Does it occur in input ports or output port? - ANS HOL blocking -
a queued packet in an input queue must wait for transfer through the fabric because it is
blocked by another packet at the head of the line. It occurs at the input port.
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 2
,R12. Do router have IP addresses? If so, how many? - ANS Yes. They have one address for
each interface.
R13. What is the 32-bit binary equivalent of the IP address 223.1.3.27? - ANS 11011111
00000001 00000011 00011100
R14. Visit a host that uses DHCP to obtain its IP address, network mask, default router, and IP
address of its local DNS server. List these values. - ANS 8 interfaces; 3 forwarding tables
R15. Suppose there are three routers between a source host and a destination host. Ignoring
fragmentation, an IP datagram sent from the source host to the destination host will travel over
how many interfaces? How many forwarding tables will be indexed to move the datagram from
the source to the destination? - ANS 50% overhead
R16. Suppose an application generates chunks of 40 bytes of data every 20 msec, and each
chunk get encapsulated in a TCP segment and then an IP datagram. What percentage of each
datagram will be overhead, and what percentage will application data? - ANS The 8-bit
protocol field in the IP datagram contains information about which transport layer protocol the
destination host should pass the segment to.
R17. Suppose Host A sends Host B a TCP segment encapsulated in an IP datagram. When Host B
receives the datagram, how does the network layer in Host B know it should pass the segment
(that is, the payload of the datagram) to TCP rather than a UDP or to something else? -
ANS Typically the wireless router includes a DHCP server. DHCP is used to assign IP addresses
to the 5 PCs and to the router interface. Yes, the wireless router also uses NAT as it obtains only
one IP address from the ISP.
R18. Suppose you purchase a wireless router and connect it to your cable modem. Also suppose
that your ISP dynamically assign your connected device (that is, your wireless router) one IP
address. Also suppose that you have five PCs at home that use 802.11 to wireless connect to
your wireless router. How are IP addresses assigned the five PCs? Does the wireless router use
NAT? Why or why not? - ANS See Section 4.4.4
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 3
, R19. Compare and contrast the IPv4 and the IPv6 header fields. Do they have any fields in
common? - ANS Yes, because the entire IPv6 datagram (including header fields) is
encapsulated in an an IPv4 datagram
R20. It has been said that when IPv6 tunnels through IPv4 router, IPv6 treats the IPv4 tunnels as
link layer protocols. Do you agree with this statement? Why or why not? - ANS Link state
algorithms: Computes the least-cost path between source and destination using complete,
global knowledge about the network. Distance-vector routing: The calculation of the least-cost
path is carried out in an iterative, distributed manner. A node only knows the neighbor to which
it should forward a packet in order to reach given destination along the least-cost path, and the
cost of that path from itself to the destination
R21. Compare and contrast link state and distance-vector routing algorithms. - ANS Key:
1.Link-state algorithms is a global routing algorithm, and it requires each node to know the cost
of each link in the network. Also, whenever a link cost changes, new link cost must be sent to all
nodes.
Distance-vector algorithm is a decentralized routing algorithm, and it requires message
exchanges between directly connected Neighbors at each iteration. When link costs change
,the DV algorithm will propagate the results of the changed link only if the new link cost result
in a changed least-cost path for one of the nodes attached to the link.
2.LS is an O(n^2)algorithm requiring O(nE)messages ,and that it potentially suffers from
oscillations. The DV algorithm can converge slowly and have routing loops .DV also suffer from
the count-to-infinity problem.
3. When a router fails, under LS, a router could broadcast an incorrect cost for one of its
attached links; under DV, a node can advertise incorrect least-cost paths to any/all destinations.
R22. Discuss how a hierarchical organization of the Internet has made it possible to scale to
millions of users. - ANS Routers are aggregated into autonomous systems (ASs). Within an
AS, all routers run the same intra-AS routing protocol. Special gateway routers in the various
ASs run the inter-autonomous system routing protocol that determines the routing paths
among the ASs. The problem of scale is solved since an intra-AS router need only know about
routers within its AS and the gateway router(s) in its AS.
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 4
QUESTIONS AND ANSWERS
R1. Let's review some of the terminology used in this textbook. Recall that the name of a
transport-layer packet is segment and that the name of a link-layer packet is frame. What is the
name of a network-layer packet? Recall that both routers and link-layer switches are called
packet switches. What is the fundamental difference between a router and link-layer switch?
Recall that we use the term routers for both datagram networks and VC networks. - ANS A
network-layer packet is a datagram. A router forwards a packet based on the packet's IP (layer
3) address. A link-layer switch forwards a packet based on the packet's MAC (layer 2) address.
R2. What are the two most important network-layer functions in a datagram network? What
are the three most important network-layer functions in a virtual-circuit network? - ANS The
two functions of datagram-based network layer are path determination and switching. The
additional function of a vc-based network is call setup.
R3. What is the difference between routing and forwarding? - ANS Forwarding is about
moving a packet from a router's input link to the appropriate output link. Routing is about
determining the end-to-routes between sources and destinations.
R4. Do the routers in both datagram networks and virtual-circuit network use forwarding
tables? If so, describe the forwarding tables for both classes of networks. - ANS Yes, both use
forwarding tables. For descriptions of the tables, see Section 4.2.
R5. Describe some hypothetical services that the network layer can provide to a single packet.
Do the same for a flow of packets. Are any of can provide to a single packet. Do the same for a
flow of packets. Are any of your hypothetical service provided by the Internet's network layer?
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 1
,Are any provided by ATM's CBR service model? Are any provided by ATM's ABR service model?
- ANS ...
R6. List some applications that would benefit from ATM's CBR service model. - ANS Single
packet: guaranteed delivery; guaranteed delivery with guaranteed maximum delay. Flow of
packets: in-order packet delivery; guaranteed minimal bandwidth; guaranteed maximum jitter.
None of these services is provided by the Internet's network layer. ATM's CBR service provides
both guaranteed delivery and timing. ABR does not provide any of these services.
R7. Discuss why each input port in a high-speed router stores shadow copy of the forwarding
table. - ANS With the shadow copy, the forwarding decision is made locally, at each input
port, without invoking the centralized routing processor. Such decentralized forwarding avoids
creating a forwarding processing bottleneck at a single point within the router.
R8. Three types of switching fabrics are discussed in Section 4.3. List and briefly describe each
type. Which, if any, can send multiple packets across the fabric in parallel. - ANS Switching
via memory; switching via a bus; switching via an interconnection network
R9. Describe how packet loss can occur at input ports. Describe how packet loss at input ports
can be eliminated (without using infinite buffers). - ANS Packet loss occurs if queue size at
the input port grows large because of slow switching fabric speed and thus exhausting router's
buffer space. It can be eliminated if the switching fabric speed is at least n times as fast as the
input line speed, where n is the number of input ports.
R10. Describe how packet loss can occur at input ports. Describe how packet loss at input ports
can be eliminated (without using infinite buffers). - ANS Packet loss can occur if the queue
size at the output port grows large because of slow outgoing line-speed.
R11. What is HOL blocking? Does it occur in input ports or output port? - ANS HOL blocking -
a queued packet in an input queue must wait for transfer through the fabric because it is
blocked by another packet at the head of the line. It occurs at the input port.
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 2
,R12. Do router have IP addresses? If so, how many? - ANS Yes. They have one address for
each interface.
R13. What is the 32-bit binary equivalent of the IP address 223.1.3.27? - ANS 11011111
00000001 00000011 00011100
R14. Visit a host that uses DHCP to obtain its IP address, network mask, default router, and IP
address of its local DNS server. List these values. - ANS 8 interfaces; 3 forwarding tables
R15. Suppose there are three routers between a source host and a destination host. Ignoring
fragmentation, an IP datagram sent from the source host to the destination host will travel over
how many interfaces? How many forwarding tables will be indexed to move the datagram from
the source to the destination? - ANS 50% overhead
R16. Suppose an application generates chunks of 40 bytes of data every 20 msec, and each
chunk get encapsulated in a TCP segment and then an IP datagram. What percentage of each
datagram will be overhead, and what percentage will application data? - ANS The 8-bit
protocol field in the IP datagram contains information about which transport layer protocol the
destination host should pass the segment to.
R17. Suppose Host A sends Host B a TCP segment encapsulated in an IP datagram. When Host B
receives the datagram, how does the network layer in Host B know it should pass the segment
(that is, the payload of the datagram) to TCP rather than a UDP or to something else? -
ANS Typically the wireless router includes a DHCP server. DHCP is used to assign IP addresses
to the 5 PCs and to the router interface. Yes, the wireless router also uses NAT as it obtains only
one IP address from the ISP.
R18. Suppose you purchase a wireless router and connect it to your cable modem. Also suppose
that your ISP dynamically assign your connected device (that is, your wireless router) one IP
address. Also suppose that you have five PCs at home that use 802.11 to wireless connect to
your wireless router. How are IP addresses assigned the five PCs? Does the wireless router use
NAT? Why or why not? - ANS See Section 4.4.4
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 3
, R19. Compare and contrast the IPv4 and the IPv6 header fields. Do they have any fields in
common? - ANS Yes, because the entire IPv6 datagram (including header fields) is
encapsulated in an an IPv4 datagram
R20. It has been said that when IPv6 tunnels through IPv4 router, IPv6 treats the IPv4 tunnels as
link layer protocols. Do you agree with this statement? Why or why not? - ANS Link state
algorithms: Computes the least-cost path between source and destination using complete,
global knowledge about the network. Distance-vector routing: The calculation of the least-cost
path is carried out in an iterative, distributed manner. A node only knows the neighbor to which
it should forward a packet in order to reach given destination along the least-cost path, and the
cost of that path from itself to the destination
R21. Compare and contrast link state and distance-vector routing algorithms. - ANS Key:
1.Link-state algorithms is a global routing algorithm, and it requires each node to know the cost
of each link in the network. Also, whenever a link cost changes, new link cost must be sent to all
nodes.
Distance-vector algorithm is a decentralized routing algorithm, and it requires message
exchanges between directly connected Neighbors at each iteration. When link costs change
,the DV algorithm will propagate the results of the changed link only if the new link cost result
in a changed least-cost path for one of the nodes attached to the link.
2.LS is an O(n^2)algorithm requiring O(nE)messages ,and that it potentially suffers from
oscillations. The DV algorithm can converge slowly and have routing loops .DV also suffer from
the count-to-infinity problem.
3. When a router fails, under LS, a router could broadcast an incorrect cost for one of its
attached links; under DV, a node can advertise incorrect least-cost paths to any/all destinations.
R22. Discuss how a hierarchical organization of the Internet has made it possible to scale to
millions of users. - ANS Routers are aggregated into autonomous systems (ASs). Within an
AS, all routers run the same intra-AS routing protocol. Special gateway routers in the various
ASs run the inter-autonomous system routing protocol that determines the routing paths
among the ASs. The problem of scale is solved since an intra-AS router need only know about
routers within its AS and the gateway router(s) in its AS.
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 4
