NWC203c Practical Complete Solutions
A university has 150 LANs with 100 hosts in each LAN.
(a) Suppose the university has one Class B address. Design an appropriate subnet addressing scheme.
(b) Design an appropriate CIDR addressing scheme. ✅a) Class B's address has 14 bits for the network ID
and 16 bits for the host ID. We need to decide how many bits to allocate to the subnet id versus the host
id for designing an appropriate subnet addressing scheme.
b) The CIDR addressing scheme involves generating a prefix length that indicates the length of the
network mask.
CÂU 4
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
Câu a) What does the router do if a packet with an IP address 135.46.63.10 arrives?
Câu b) What does the router do if a packet with an IP address 135.46.57.14 arrives? ✅Cách nhận biết:
Xét 135.46.63.10 có 135.46 giống Interface 0 và 1.
135.46.63 lớn hơn Interface 1 thì chọn Interface 1, ngược nếu ví dụ như đề câu b chỉ có 57 (lớn hơn 56
nhưng nhỏ hơn 60) -> chọn Interface 0.
, Nếu đề hỏi khác nữa như cho 10.10.10.10 không giống cái nào ở Interface 0, 1 hay Router 1 thì mặc định
chọn Default -> Router 2.
CÂU 3
Consider the three-way handshake in TCP connection setup.
a) Suppose that an old SYN segment from station A arrives at station B, requesting a TCP connection.
Explain how the three-way handshake procedure ensures that the connection is rejected.
b)Now suppose that an old SYN segment from station A arrives at station B, followed a bit later by an old
ACK segment from A to a SYN segment from B. Is this connection request also rejected? ✅a) In a three-
way handshake procedure, one must ensure the selection of the initial sequence number is always
unique. If station B receives an old SYN segment from A, B will acknowledge the request based on the
old sequence number. When A receives the acknowledgment segment from B, A will find out that B
received a wrong sequence number. A will discard the acknowledgment packet and reset the
connection.
b) If an old SYN segment from A arrives at B, followed by an old ACK segment from A to a SYN segment
from B, the connection will also be rejected. Initially, when B receives an old SYN segment, B will send a
SYN segment with its own distinct sequence number set by itself. If B receives the old ACK from A, B will
notify A that the connection is invalid since the old ACK sequence number does not match the sequence
number previously defined by B. Therefore, the connection is rejected.
A bit stream 1101011011 is transmitted using the standard CRC method. The polynomial generator is x+
x + 1. What is the actual bit string transmitted? Show the major steps to your answer. ✅Bitstream is
1101011011
The polynomial generator is x+x=1
The Bit form = 111
Thus, n=3.
Suppose a IP header consists of four 16-bit words: (11111111 11111111, 11111111 00000000, 11110000
11110000, 11000000 11000000). Please find the Internet checksum for the code. ✅Đổi cái đám nhị
nhân 16 bit ra decimal, ra số âm xong lấy 65536 (16 bit thì là 2^16) rồi cộng cho số âm đó.
A university has 150 LANs with 100 hosts in each LAN.
(a) Suppose the university has one Class B address. Design an appropriate subnet addressing scheme.
(b) Design an appropriate CIDR addressing scheme. ✅a) Class B's address has 14 bits for the network ID
and 16 bits for the host ID. We need to decide how many bits to allocate to the subnet id versus the host
id for designing an appropriate subnet addressing scheme.
b) The CIDR addressing scheme involves generating a prefix length that indicates the length of the
network mask.
CÂU 4
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
Câu a) What does the router do if a packet with an IP address 135.46.63.10 arrives?
Câu b) What does the router do if a packet with an IP address 135.46.57.14 arrives? ✅Cách nhận biết:
Xét 135.46.63.10 có 135.46 giống Interface 0 và 1.
135.46.63 lớn hơn Interface 1 thì chọn Interface 1, ngược nếu ví dụ như đề câu b chỉ có 57 (lớn hơn 56
nhưng nhỏ hơn 60) -> chọn Interface 0.
, Nếu đề hỏi khác nữa như cho 10.10.10.10 không giống cái nào ở Interface 0, 1 hay Router 1 thì mặc định
chọn Default -> Router 2.
CÂU 3
Consider the three-way handshake in TCP connection setup.
a) Suppose that an old SYN segment from station A arrives at station B, requesting a TCP connection.
Explain how the three-way handshake procedure ensures that the connection is rejected.
b)Now suppose that an old SYN segment from station A arrives at station B, followed a bit later by an old
ACK segment from A to a SYN segment from B. Is this connection request also rejected? ✅a) In a three-
way handshake procedure, one must ensure the selection of the initial sequence number is always
unique. If station B receives an old SYN segment from A, B will acknowledge the request based on the
old sequence number. When A receives the acknowledgment segment from B, A will find out that B
received a wrong sequence number. A will discard the acknowledgment packet and reset the
connection.
b) If an old SYN segment from A arrives at B, followed by an old ACK segment from A to a SYN segment
from B, the connection will also be rejected. Initially, when B receives an old SYN segment, B will send a
SYN segment with its own distinct sequence number set by itself. If B receives the old ACK from A, B will
notify A that the connection is invalid since the old ACK sequence number does not match the sequence
number previously defined by B. Therefore, the connection is rejected.
A bit stream 1101011011 is transmitted using the standard CRC method. The polynomial generator is x+
x + 1. What is the actual bit string transmitted? Show the major steps to your answer. ✅Bitstream is
1101011011
The polynomial generator is x+x=1
The Bit form = 111
Thus, n=3.
Suppose a IP header consists of four 16-bit words: (11111111 11111111, 11111111 00000000, 11110000
11110000, 11000000 11000000). Please find the Internet checksum for the code. ✅Đổi cái đám nhị
nhân 16 bit ra decimal, ra số âm xong lấy 65536 (16 bit thì là 2^16) rồi cộng cho số âm đó.
