# Networking

**Paper, Order, or Assignment Requirements**

Log inID- ys223 please answer the questions below, where X and Y are last two numbers of your login ID, eg. if your login is abc356 X=5 and Y=6. If your login has only one number then X=0, if you have no numbers in your login X=Y=0. As an example if your login is abc7 then X=0 and Y=7; if your login is abc then X=0 and Y=0:1) Suppose an ISP owns the block of addresses of the form 204.0.0.0/k where k=X+Y+7. Suppose it wants to create four subnets from this block, with each block having the same number of IP addresses What are the prefixes (of form a.b.c.d/h) for the four subnets? Justify your answer.

2) Suppose the ISP in question 1 would like to change the partitioning of its block of addresses, e.g subdividing the block in 8 subnets. Does this change need to be advertised to other networks? Explain your reasoning. Suppose the addresses of one of the subnet were used by a company, client of the ISP. What happens if the company wants to change ISP?

[6 marks]

3) Suppose an IP packet of 490+80*(X+Y) bytes on network N1 travels on network N2 with MTU of 192+24*(X+Y) bytes and then on network N3 with an MTU of 88+16*(X+Y) bytes. Show the content of the fields of the first 8 bytes of each IP packet received at the destination (for example if your login is abc12 the original IP packet can be described as ver = 4, hlen= 20, type=T, length= 480, id=X, fflag=0, foffset=0)

4) Consider a network with link costs c(a,b)= 30 – 3*(X-Y), c(b,c) = 2+X, c(b,d) = 10+X, c(a,d) = 80+X+Y, c(c,d)=1, c(b,e)=20+Y, c(a,e)=33+3*(X-Y), c(c,e)=20+Y, c(d,e)=1+Y. Simulate the execution of the Dijkstra’s algorithm starting from node a, filling in Table 1.

[8 marks]

5) Suppose that three switches are used to connect 10 hosts (h_0, h_1, …, h_9). Assume switch s_1 is connected to h_0,h_1,h_2,h_3, and switch s_2 (using interfaces 1 to 5 respectively); s_2 is connected to hosts h_4, h_5, h_6 and switch s_1 and s_3 (interfaces 1 to 5 respectively); and s_3 is connected to hosts h_7,h_8,h_9, and switch s_2 (on interfaces 1 to 4 respectively). Consider the transmission of a frame from host h_(X+Y mod 10) to host h_(X+Y+6 mod 10) and a reply back to the source.

a) Assuming the switch tables start empty, simulate the execution of the self-learning forwarding protocol by representing each transmission with the following notation: “Ti: sender -> (dest1, dest2, …)” where sender is the sending host or switch, dest1..destn are the set of destinations to which the message is sent, and T0, T1, … indicate increasing instants in time. For example a first transmission from h_0 to s_1 would be indicated with: “T0: h_0->(s_1)”.

b) Show the content of the tables for the three switches at the end of the execution, with the notation “S_i={(host1,interf1,T1), (host2, interf2, T2), …}”, e.g., s_1={(h_0,1,T0)} indicates switch s_1 learned that host h_0 is reachable through interface 1 at time T0.

[5 marks]

6) Suppose host A needs to send a message to B whose content in hexadecimal is your login (lowercase), coded in UTF8. Compute the parity bits according to the following schemes: a) single bit odd parity, b) column (odd) parity scheme with 4 and 8 columns c) two dimensional parity schemes with 8 columns. Then say how many errors can be detected or corrected with each scheme, and compute the overhead of the scheme i.e., the ratio between the number of bits used for error correction and the number of data bits.

[6 marks]

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