Question $2$

$($a$)$ The IPv$4$ datagram is shown in Figure $2\mathrm{.}1$ below. The bits allocated to each field are

shown at the top of the frame.

Figure $2\mathrm{.}1$

$($i$)$ Calculate the size of the Payload in the transmitted packet, if the entry of the IHL

field in the header is $1010$ and the Total length entry is $0000010000010000.$

$($ii$)$ For the same packet as in part $($i$)$ find how many bytes were put into the Options

field.

$($iii$)$ Is it possible from the information provided in part $($i$)$ to define whether any padding

was used in the packet? If not, explain why. If yes, how big was the padding?

$($b$)$ The IPv$4$ addressing is based on a relatively small $32-$bit address space, which requires to

utilize various schemes for an efficient address allocation.

$($i$)$ The administrator of a campus LAN is assigned a single IP address of $172\mathrm{.}16\mathrm{.}0\mathrm{.}0.$

Assuming that a LAN should comprise at least $100$ subnets and each subnet should

contain at most $50$ hosts, define a suitable subnet mask for the site. Find how many

subnets and how many hosts per subnet will be allowed.

$($ii$)$ The ISP $($Internet Service Provider$)$ has allocated the following block of IP addresses

to two networks which use CIDR $($Classless Inter$-$Domain Routing$)$ addressing:

Network $1$: IP$1$ Base Address $=240\mathrm{.}80\mathrm{.}16\mathrm{.}0/20$

Mask M$1=255\mathrm{.}255\mathrm{.}240\mathrm{.}0$

Network $2$: IP$2$ Base Address $=240\mathrm{.}80\mathrm{.}31\mathrm{.}0/24$

Mask M$2=255\mathrm{.}255\mathrm{.}255\mathrm{.}0$

If a packet with the IP destination address IP$0=240\mathrm{.}80\mathrm{.}31\mathrm{.}155$ arrives at the router

to which the Networks $1$ and $2$ are attached, carry out a Boolean logical AND

operation on IP$0$ and each of the address masks M$1$ and M$2,$ and compare the

obtained result with the given base addresses IP$1$ and IP$2.$ To which network the

packet will be routed? Explain why.

$($c$)$ An IPv$4$ link shown in Figure $2\mathrm{.}2$ consists of two hosts H$1$ and H$2$ and a router R$.$ The

router R defines the link$$s maximum transmission unit $($MTU$)$ MTU $=1500$ bytes. The

application layer of the host H$1$ generates a $1480-$byte file which is given to the transport

layer for a reliable transmission to the host H$2.$ The network uses a non$-$transparent

fragmentation.

Figure $2\mathrm{.}2$

$($i$)$ Is fragmentation of the packet required in this case? Explain your answer.

$($ii$)$ In case the packet is fragmented, calculate the payload of each fragment.

$($iii$)$ Show the Total Length, DF$,$ MF$,$ and Fragment Offset fields of the IP header in each

packet transmitted over this link. Explain why these fields are needed in the packet

fragmentation.

$($d$)$ Compare the OSI and the TCP$/$IP reference models, sketch their layer structure, and

explain their similarities and differences. $[$Note$,$ that you are not required to explain the

functionality of each layer in the models$].$