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杭州电子科技大学

《计算机与网络实验》

课程实验报告

实验六 :DNS, Data Encapsulation and Frame

Examination

学 院 通信工程 班 级 14083415 学 号 14081515 姓 名 何必锋 指导教师 冯维

杭州电子科技大学通信工程学院

Learning Objectives

At completion of this lab, you will be able to:

1. Use Wireshark to capture and analyze DNS message 2. Understand how DNS works 3. use nslookup and ipconfig commands

4. Explain the header fields in an Ethernet II frame. 5. understand data encapsulation

6. Report and Feedback on this lab

Answer all questions with supporting screenshots. Please fill in the following feedback form and append it to the report. Your feedback is valuable to us so that we can improve this lab, and make the course welcome.

Background

When upper layer protocols communicate with each other, data at the sending host flows down the TCP/IP protocol layers and is encapsulated into a protocol data unit at lower layer, and finally encapsulated in a Layer 2 frame. For example, DNS message is often transported by UDP protocol on layer 4. So DNS message at the sending host is encapsulated in a UDP segment; the UDP segment is then encapsulated in an IP packet, and the IP packet is encapsulated finally in a layer 2 frame. The frame composition is dependent on the media access type, or the network. For example, if the media access is Ethernet, then the Layer 2 frame encapsulation will be Ethernet II.

When learning about data encapsulation and protocol operations, it is helpful to analyze the header information found in the protocol data units. The DNS protocol operation, ipconfig command, and Ethernet II frame header will be examined in this lab. Ethernet II frames can support various upper layer protocols.

Tasks

Task 0 Protocol Layers and Data Encapsulation

As we have discussed in Lecture 02, Internet is inter-connected networks based on TCP/IP protocols. Read the slides or textbook to learn how data goes through protocol layers and how data is encapsulated in the protocol data units. There are conventional names for the protocol data units for different layer protocols.

Question 1. What are names for the protocol data units (PDUs) for layer 4, layer 3, and layer 2 protocols in TCP/IP reference model by filling the follow form:

对协议数据单元（PDU）的名字都是什么4层、3层和2层协议，TCP/IP参考模型填充下列表格：

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杭州电子科技大学通信工程学院

name for PDU of layer 4 protocols: segment

name for PDU of layer 3 protocol: packet

name for PDU of layer 2 protocol: frame

Task 1 DNS and nslookup

As we discussed, IP address is used to identify a host uniquely on the Internet. But IP address is not user-friendly and that is why domain name was introduced. The Domain Name System (DNS) translates host names to IP addresses, providing a critical role in the Internet infrastructure.

In this task, we practice the nslookup tool, which is available both in Linux/Unix and MS Windows. To run nslookup in MS Windows, you need to open the command line window by starting the command \ With nslookup, you can query any specified DNS server (by default, your local configured DNS server) for a DNS record. To accomplish this task, nslookup sends a DNS query to the specified DNS server, receives a DNS reply from that same DNS server, and displays the result.

Type the command \ www.MIT.edu\

showed below:

Question 2: What is the DNS server IP address that is used to query and find IP address for www.MIT.edu? And IP address for

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