NOCTI COMPUTER NETWORKING TEST PAPER 2025/2026
QUESTIONS AND SOLUTIONS MARKED A+
✔✔R11. What is HOL blocking? Does it occur in input ports or output port? - ✔✔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.
✔✔R12. Do router have IP addresses? If so, how many? - ✔✔Yes. They have one
address for each interface.
✔✔R13. What is the 32-bit binary equivalent of the IP address 223.1.3.27? -
✔✔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. - ✔✔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? - ✔✔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? - ✔✔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? - ✔✔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? - ✔✔See Section 4.4.4
,✔✔R19. Compare and contrast the IPv4 and the IPv6 header fields. Do they have any
fields in common? - ✔✔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?
- ✔✔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. -
✔✔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. - ✔✔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.
✔✔R23. Is it necessary that every autonomous system use the same intra-AS routing
algorithm? Why or why not? - ✔✔No. Each AS has administrative autonomy for routing
within an AS.
✔✔R24. Consider Figure 4.37. Starting with the original table in D, suppose that D
receives from A the following advertisement:
Destination Subnet Next Router Number of Hops to Destination
z c 10
,w __ 1
x __ 1
.........
Will the table in A change? If so how? - ✔✔No. The advertisement tells D that it can get
to z in 11 hops by way of A. However, D can already get to z by way of B in 7 hops.
Therefore, there is no need to modify the entry for z in the table. If, on the other hand,
the advertisement said that A were only 4 hops away from z by way of C, then D would
indeed modify its forwarding table.
✔✔R25. Compare and contrast the advertisements used by RIP and OSPF. - ✔✔With
OSPF, a router periodically broadcasts routing information to all other routers in the AS,
not just to its neighboring routers. This routing information sent by a router has one
entry for each of the router's neighbors; the entry gives the distance from the router to
the neighbor. A RIP advertisement sent by a router contains information about all the
networks in the AS, although this information is only sent to its neighboring routers.
✔✔R26. Fill in the blank: RIP advertisements typically announce the number of hops to
various destinations. BGP updates, on the other hand, announce the __________ to the
various destinations. - ✔✔"sequence of ASs on the routes"
✔✔R27. Why are different inter-AS and intra-AS protocols used in the Internet? -
✔✔See "Principles in Practice" on page 384
✔✔R28. Why are policy considerations as important for intra-AS protocols, such as
OSPF and RIP, as they are for an inter-AS routing protocol like BGP? - ✔✔ISP C can
use the BGP Multi-Exit Descriptor to suggest to ISP B that the preferred route to ISP D
is through the east coast peering point. For example, the east coast BGP router in ISP
C can advertise a route to D with an MED value of 5. The west coast router in ISP C
can advertise a route to D with an MED value of 10. Since a lower value is preferred,
ISP B knows that ISP C wants to receive traffic on the east coast. In practice, a router
can ignore the MED value, and so ISP B can still use hot potato routing to pass traffic to
ISP C destined to ISP D via the west coast peering point.
✔✔R29. Define and contrast the following terms: subnet, prefix, and BGP route. - ✔✔A
subnet is a portion of a larger network; a subnet does not contain a router; its
boundaries are defined by the router and host interfaces. A prefix is the network portion
of a CDIRized address; it is written in the form a.b.c.d/x ; A prefix covers one or more
subnets. When a router advertises a prefix across a BGP session, it includes with the
prefix a number of BGP attributes. In BGP jargon, a prefix along with its attributes is a
BGP route (or simply a route).
✔✔R30. How does BGP use the NEXT-HOP attribute? How does it use the AS-PATH
attribute? - ✔✔Routers use the AS-PATH attribute to detect and prevent looping
advertisements; they also use it in choosing among multiple paths to the same prefix.
, The NEXT-HOP attribute indicates the IP address of the first router along an advertised
path (outside of the AS receiving the advertisement) to a given prefix. When configuring
its forwarding table, a router uses the NEXT-HOP attribute.
✔✔R31. Describe how a network administrator of an upper-tier ISP can implement
policy when configuring BGP. - ✔✔A tier-1 ISP B may not to carry transit traffic between
two other tier-1 ISPs, say A and C, with which B has peering agreements. To implement
this policy, ISP B would not advertise to A routes that pass through C; and would not
advertise to C routes that pass through A.
✔✔R32. What is an important difference between implementing the broadcast
abstraction via multiple unicasts, and a single network - (router-) supported broadcast? -
✔✔N-way unicast has a number of drawbacks, including:
• Efficiency: multiple copies of the same packet are sent over the same link for
potentially many links; source must generate multiple copies of same packet
• Addressing: the source must discover the address of all the recipients
✔✔R33. For each of the three general approaches we studied for broadcast
communication (uncontrolled flooding, controlled flooding, and spanning-tree
broadcast), are the following statement true or false? You may assume that no packets
are lost due to buffer overflow and all packets are delivered on a link in the order in
which they were sent.
a. A node may receive multiple copies of the same packet.
b. A node may forward multiple copies of a packet over the same outgoing link. - ✔✔32.
a) uncontrolled flooding: T; controlled flooding: T; spanning-tree: F
b) uncontrolled flooding: T; controlled flooding: F; spanning-tree: F
✔✔R34. When a host joins a multicast group, must it change its IP address to that of
the multicast group it is joining? - ✔✔False
✔✔R35. What are the roles played by IGMP protocol and a wide-area multicast routing
protocol? - ✔✔IGMP is a protocol run only between the host and its first-hop multicast
router. IGMP allows a host to specify (to the first-hop multicast router) the multicast
group it wants to join. It is then up to the multicast router to work with other multicast
routers (i.e., run a multicast routing protocol) to ensure that the data for the host-joined
multicast group is routed to the appropriate last-hop router and from there to the host.
✔✔R36. What is the difference between a group-shared tree and a source-based tree in
the context of multicast routing? - ✔✔In a group-shared tree, all senders send their
multicast traffic using the same routing tree. With source-based tree, the multicast
datagrams from a given source are routed over s specific routing tree constructed for
that source; thus each source may have a different source-based tree and a router may
have to keep track of several source-based tress for a given multicast group.
QUESTIONS AND SOLUTIONS MARKED A+
✔✔R11. What is HOL blocking? Does it occur in input ports or output port? - ✔✔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.
✔✔R12. Do router have IP addresses? If so, how many? - ✔✔Yes. They have one
address for each interface.
✔✔R13. What is the 32-bit binary equivalent of the IP address 223.1.3.27? -
✔✔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. - ✔✔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? - ✔✔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? - ✔✔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? - ✔✔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? - ✔✔See Section 4.4.4
,✔✔R19. Compare and contrast the IPv4 and the IPv6 header fields. Do they have any
fields in common? - ✔✔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?
- ✔✔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. -
✔✔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. - ✔✔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.
✔✔R23. Is it necessary that every autonomous system use the same intra-AS routing
algorithm? Why or why not? - ✔✔No. Each AS has administrative autonomy for routing
within an AS.
✔✔R24. Consider Figure 4.37. Starting with the original table in D, suppose that D
receives from A the following advertisement:
Destination Subnet Next Router Number of Hops to Destination
z c 10
,w __ 1
x __ 1
.........
Will the table in A change? If so how? - ✔✔No. The advertisement tells D that it can get
to z in 11 hops by way of A. However, D can already get to z by way of B in 7 hops.
Therefore, there is no need to modify the entry for z in the table. If, on the other hand,
the advertisement said that A were only 4 hops away from z by way of C, then D would
indeed modify its forwarding table.
✔✔R25. Compare and contrast the advertisements used by RIP and OSPF. - ✔✔With
OSPF, a router periodically broadcasts routing information to all other routers in the AS,
not just to its neighboring routers. This routing information sent by a router has one
entry for each of the router's neighbors; the entry gives the distance from the router to
the neighbor. A RIP advertisement sent by a router contains information about all the
networks in the AS, although this information is only sent to its neighboring routers.
✔✔R26. Fill in the blank: RIP advertisements typically announce the number of hops to
various destinations. BGP updates, on the other hand, announce the __________ to the
various destinations. - ✔✔"sequence of ASs on the routes"
✔✔R27. Why are different inter-AS and intra-AS protocols used in the Internet? -
✔✔See "Principles in Practice" on page 384
✔✔R28. Why are policy considerations as important for intra-AS protocols, such as
OSPF and RIP, as they are for an inter-AS routing protocol like BGP? - ✔✔ISP C can
use the BGP Multi-Exit Descriptor to suggest to ISP B that the preferred route to ISP D
is through the east coast peering point. For example, the east coast BGP router in ISP
C can advertise a route to D with an MED value of 5. The west coast router in ISP C
can advertise a route to D with an MED value of 10. Since a lower value is preferred,
ISP B knows that ISP C wants to receive traffic on the east coast. In practice, a router
can ignore the MED value, and so ISP B can still use hot potato routing to pass traffic to
ISP C destined to ISP D via the west coast peering point.
✔✔R29. Define and contrast the following terms: subnet, prefix, and BGP route. - ✔✔A
subnet is a portion of a larger network; a subnet does not contain a router; its
boundaries are defined by the router and host interfaces. A prefix is the network portion
of a CDIRized address; it is written in the form a.b.c.d/x ; A prefix covers one or more
subnets. When a router advertises a prefix across a BGP session, it includes with the
prefix a number of BGP attributes. In BGP jargon, a prefix along with its attributes is a
BGP route (or simply a route).
✔✔R30. How does BGP use the NEXT-HOP attribute? How does it use the AS-PATH
attribute? - ✔✔Routers use the AS-PATH attribute to detect and prevent looping
advertisements; they also use it in choosing among multiple paths to the same prefix.
, The NEXT-HOP attribute indicates the IP address of the first router along an advertised
path (outside of the AS receiving the advertisement) to a given prefix. When configuring
its forwarding table, a router uses the NEXT-HOP attribute.
✔✔R31. Describe how a network administrator of an upper-tier ISP can implement
policy when configuring BGP. - ✔✔A tier-1 ISP B may not to carry transit traffic between
two other tier-1 ISPs, say A and C, with which B has peering agreements. To implement
this policy, ISP B would not advertise to A routes that pass through C; and would not
advertise to C routes that pass through A.
✔✔R32. What is an important difference between implementing the broadcast
abstraction via multiple unicasts, and a single network - (router-) supported broadcast? -
✔✔N-way unicast has a number of drawbacks, including:
• Efficiency: multiple copies of the same packet are sent over the same link for
potentially many links; source must generate multiple copies of same packet
• Addressing: the source must discover the address of all the recipients
✔✔R33. For each of the three general approaches we studied for broadcast
communication (uncontrolled flooding, controlled flooding, and spanning-tree
broadcast), are the following statement true or false? You may assume that no packets
are lost due to buffer overflow and all packets are delivered on a link in the order in
which they were sent.
a. A node may receive multiple copies of the same packet.
b. A node may forward multiple copies of a packet over the same outgoing link. - ✔✔32.
a) uncontrolled flooding: T; controlled flooding: T; spanning-tree: F
b) uncontrolled flooding: T; controlled flooding: F; spanning-tree: F
✔✔R34. When a host joins a multicast group, must it change its IP address to that of
the multicast group it is joining? - ✔✔False
✔✔R35. What are the roles played by IGMP protocol and a wide-area multicast routing
protocol? - ✔✔IGMP is a protocol run only between the host and its first-hop multicast
router. IGMP allows a host to specify (to the first-hop multicast router) the multicast
group it wants to join. It is then up to the multicast router to work with other multicast
routers (i.e., run a multicast routing protocol) to ensure that the data for the host-joined
multicast group is routed to the appropriate last-hop router and from there to the host.
✔✔R36. What is the difference between a group-shared tree and a source-based tree in
the context of multicast routing? - ✔✔In a group-shared tree, all senders send their
multicast traffic using the same routing tree. With source-based tree, the multicast
datagrams from a given source are routed over s specific routing tree constructed for
that source; thus each source may have a different source-based tree and a router may
have to keep track of several source-based tress for a given multicast group.
