School of Computing and Information Systems
COMP30023: Computer Systems
Tutorial Week 11
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Network Layer – IP Addresses and Packet Switching
Note: This lab sheet is longer than will fit into one tutorial time. That is to provide you with
extra study material when preparing for the exam.
It is up to you and your tutor to agree which questions to cover in the tutorial time.
1. Do routers have IP addresses? If so, how many?
2. Datagram networks route each packet as a separate unit, independent of all others. Virtual-
circuit networks do not have to do this, since each data packet follows a predetermined route.
Does this observation mean that virtual-circuit networks do not need the capability to route
isolated packets from an arbitrary source to an arbitrary destination? Explain your answer.
3. A network on the Internet has a subnet mask of 255.255.240.0. What is the maximum
number of hosts it can handle?
4. A router has just received routes to the following new IP networks: 57.6.96.0/21, 57.6.104.0/21,
57.6.112.0/21, and 57.6.120.0/21. If all of them use the same outgoing line, can they be ag-
gregated? If so, to what? If not, why not?
5. A large number of consecutive IP addresses are available starting at 198.16.0.0. Suppose
that four organizations, A, B, C, and D, request 4000, 2000, 4000, and 8000 addresses,
respectively, and in that order. Each request is given the lowest-numbered subnet available
for it. For each of these, give the first IP address assigned, the last IP address assigned, and
the mask in the w.x.y.z/s notation.
6. A router has the following (CIDR) entries in its routing table:
Address/mask Next hop
135.46.56.0/22 Interface 0
135.46.60.0/22 Interface 1
192.53.40.0/23 Router 1
default Router 2
For each of the following IP addresses, where does the router send the packet if a packet
with that address arrives?
(a) 135.46.63.10
(b) (for exam study: 135.46.57.14)
(c) (for exam study: 135.46.52.2)
(d) (for exam study: 192.53.40.7)
(e) (for exam study: 192.53.56.7)
7. List one motivation for a host to send an IP packet with the wrong source IP address. List
two ways that this can adversely affect the legitimate owner of that IP address.
8. The IP packet header includes a time-to-live field that is decremented by each router along
the path. Why is the time-to-live field necessary?
9. Bonus for fun: IPv6 uses 16-byte addresses. If a block of 1 million addresses is allocated
every picosecond, how long will the addresses last?
Weekly tutorial participation activity
To obtain a weekly tutorial mark (1% of your overall mark for the subject, totalling to 10% over
the semester), please answer the following questions in Canvas Quiz (called Week 11 Tutorial
Activity). You can have multiple attempts but need to submit the quiz by 11:59 pm AEST on
the day of your tutorial. Only answering all questions correctly will give you the week’s mark.
During the tutorial, your tutor will provide you with the access code that will unlock the quiz for
the corresponding week. The access code is valid only for students in this tutorial. As a result,
we expect you to attend your tutorial class as otherwise the access code you obtain in another
tutorial will not work for you.
If you have a valid reason for not being able to attend your tutorial, please fill in the form accessible
via Canvas by Friday 8pm AEDT of the corresponding week. You will be given an access code
during the next business day. The code will not be provided otherwise, hence, please do not email
the subject coordinator or other staff members including your tutor asking for the code. We will
monitor such requests and may limit the number of times an access code is given to the same
student throughout the semester to encourage participation and attendance of the tutorials.
1. What is the smallest network (or subnet) that contains both 128.50.4.8 and 128.50.3.254?
(a) 128.50.3.254/28
(b) 128.50.0.0/21
(c) 128.50.0.0/20
(d) 128.50.0.0/16
2. If the time to live field of a paket contains the number 2 when it arrives at a router, and it
spends 1.5 seconds queued at that router, which of the following is correct?
(a) It can only spend 0.5 seconds in the next router before being dropped
(b) When it arrives at the next device (router or host), it will have a TTL of 1
(c) If the next device is a router connected to the final destination, then the router will
decrement the TTL to 0 and then forward it to the host
(d) When the packet is forwarded to the next route, it will have a TTL value of 0.5
3. Of the 16 bytes of an IPv6 address, 8 bytes are used for the network number. How many
networks per person on earth does this allow?
(a) 28 = 256
(b) 28/(9e9) ≈ 2−8
(c) 28×8/(9e9) ≈ 2e9
(d) 168/(9e9) ≈ 0.48
4. In Question 5, which of the following is true?
(a) Organization A could request another 90 IP addresses without requiring any reconfig-
uration, but organization B could not
(b) Organization B could request another 1000 IP addresses without requiring any recon-
figuration, but organization A could not
(c) If Organization D required 200 more addresses, then they could not all receive contigu-
ous allocations
(d) Any change in requests would require reconfiguration
5. Which of the following subnet addresses could have subnet mask of 255.255.240.0?
(a) 1.1.1.0
(b) 1.1.128.0
(c) 128.127.126.0
(d) 9.10.11.12
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