实验一Ethernet and ARP一、实验目的1.加强对以太网帧格式的理解；2.理解ARP协议的工作原理。

二、实验环境1.PC机一台；2.WireShark软件。

三、实验内容1. Capturing and analyzing Ethernet frames2. The Address Resolution Protocol四、实验步骤及思考（一）Capturing and analyzing Ethernet framesHTTP GET Message(1)Based on the contents of the Ethernet frame containing the HTTP GET message1. What is the 48-bit Ethernet address of your computer?Answer：The 48-bit Ethernet address of my computer is:c8:0a:a9:db:9b:f3The Ethernet address of my computer2. What is the 48-bit destination address in the Ethernet frame? Is this the Ethernet address of ? (Hint: the answer is no). What device has this as its Ethernet address?[Note: this is an important question, and one that students sometimes get wrong. Re-read pages 468-469 in the text and make sure you understand the answer here.]Answer：1)The 48-bit destination address in the Ethernet frame is:00:23:89:8d:50:71The destination address in the Ethernet frame2)This is not the Ethernet address of .3)It is the address of my Linksys router, whick is the link used to get off the subnet.3.Give the hexadecimal value for the two-byte Frame type field. What do the bit(s) whose value is 1 mean within the flag field?Answer：1)The hexadecimal value for the two-byte Frame type field is:ox0800The hexadecimal value forthe two-byte Frame type2) The value is 1 within the flag field means the fragment has not been fragmented.Flag4.How many bytes from the very start of the Ethernet frame does the ASCII “G” in“GET” appear in the Ethernet frame?Answer：1)T he ASCII “G” appears 54 bytes from the very start of the Ethernet frame.2)Calculation process as follows:There are 14 bytes of Ethernet frame,20 bytes of IP header and 20 bytes of TCP header before HTTP data is encountered.5. What is the hexadecimal value of the CRC field in this Ethernet frame?Answer：1)There is no CRC field.2)Because the CRC calculated before the Wireshark packet sniffer start up.(2)Based on the contents of the Ethernet frame containing the first byte of the HTTP response message6. What is the value of the Ethernet source address? Is this the address of your computer, or of (Hint: the answer is no). What device has this as its Ethernet address?Answer ：1)The value of the Ethernet source address is: 00:23:89:8d:50:712)This is neither the address of your computer,northe address of .3)It is the address of my Linksys router,which is the link used to get onto HTTP response messagemy subnet.7. What is the destination address in the Ethernet frame? Is this the Ethernet address of your computer?Answer：1)The destination address in the Ethernet frame is:c8:0a:a9:db:9b:f32)It is the address of my computer.(The 48-bit Ethernet address of my computer is:c8:0a:a9:db:9b:f3)8. Give the hexadecimal value for the two-byte Frame type field. What do the bit(s) whose value is 1 mean within the flag field?Answer：1)The hexadecimal value for the two-byte Frame type field is: 0x0800.2)The value is 1 within the flag field means the fragment has not been fragmented.Flag field9. How many bytes from the very start of the Ethernet frame does the ASCII “O” in“OK” (i.e., the HTTP response code) appear in the Ethernet frame?Answer：1)T he ASCII “O” appears 54 bytes from the very start of the Ethernet frame.2)Calculation process as follows:There are 14 bytes of Ethernet frame,20 bytes of IP header and 20 bytes of TCP header before HTTP data is encountered.10. What is the hexadecimal value of the CRC field in this Ethernet frame?Answer：1)There is no CRC field.2)The reason:Because the CRC calculated before the Wireshark packet sniffer start up.(二)The Address Resolution Protocol（1）ARP Caching11. Write down the contents of your computer’s ARP cache. What is the meaning of each column value?Answer：1)The Internet Address column contains the IP address,.2)The Physical Address column contains the MAC address, and the type indicates the protocol type.3)Observing ARP in action12. What are the hexadecimal values for the source and destination addresses in the Ethernet frame containing the ARP request message? Answer：1)The hexadecimal values for the source addresses in the Ethernet frame containing the ARP request message is: 00:23:89:8d:50:712)The destination addresses in the Ethernet frame containing the ARP request message is: ff:ff:ff:ff:ff:ff13. Give the hexadecimal value for the two-byte Ethernet Frame type field. What do the bit(s) whose value is 1 mean within the flag field? Answer：The hexadecimal value for the two-byte Ethernet Frame type field is:0x0806.14.Download the ARP specification from . A readable, detailed discussion of ARP is also at .a) How many bytes from the very beginning of the Ethernet frame does the ARP opcode field begin?Answer：The ARP opcode field begins 20 bytes from the very beginning ofthe Ethernet frame.b) What is the value of the opcode field within the ARP-payload part of the Ethernet frame in which an ARP request is made?Answer：The hex value for opcode field withing the ARP-payload of the request is 1.c) Does the ARP message contain the IP address of the sender? Answer：Yes, the ARP message containg the IP address 192.168.1.105 for the sender.d) Where in the ARP request does the “question” appear – the Ethernet address of the machine whose corresponding IP address is being queried?Answer：The field “Target MAC address” is set to 00:00:00:00:00:00 to question the machine whose corresponding IP address (192.168.1.105) is being queried.15.Now find the ARP reply that was sent in response to the ARP request.a) How many bytes from the very beginning of the Ethernet frame does the ARP opcode field begin?Answer：The ARP opcode field begins 20 bytes from the very beginning of the Ethernet frame.b) What is the value of the opcode field within the ARP-payload part of the Ethernet frame in which an ARP response is made?Answer：The hex value for opcode field withing the ARP-payload part of the Ethernet frame is 2．c) Where in the ARP message does the “answer” to the earlier ARP request appear –the IP address of the machine having the Ethernet address whose corresponding IP address is being queried?Answer：The answer to the earlier ARP request appears in the”Sender MAC address”field, which contains the Ethernet address 00:d0:59:a9:3d:68 for the sender with IP address 192.168.1.1.16. What are the hexadecimal values for the source and destination addresses in the Ethernet frame containing the ARP reply message? Answer：The hex value for the source address is 00:06:25:da:af:73 and for the destination is 00:d0:59:a9:3d:68 .17. Open the ethernet-ethereal-trace-1 trace. The first and second ARP packets in this trace correspond to an ARP request sent by the computer running Wireshark, and the ARP reply sent to the computer running Wireshark by the computer with the ARP-requested Ethernet address. But there is yet another computer on this network, as indiated by packet 6 –another ARP request. Why is there no ARP reply (sent in response to the ARP request in packet 6) in the packet trace?Answer：There is no reply in this trace, because we are not at the machinethat sent the request. The ARP request is broadcast, but the ARP reply is sent back directly to the sender’s Ethernet address.五、实验结论1.CRC在WireShark中是不能被抓到的，原因是CRC在抓包之前已经被计算好。