The LAN automatically obtains IP addresses and always conflicts.

IP conflicts occur mainly for the following reasons. Some people may not know much about TCP/IP and how to set the parameters such as "IP address", "Subnet mask" and "Default gateway" (don't laugh at them, after all, we did it step by step), there are The parameter information they get may not be what the administrator told them, or it may be correct at the beginning but then unintentionally and accidentally modify the information. Even if the administrator gives correct parameter data, we always make mistakes. Because the tiger is still taking a nap, it is inevitable to enter the wrong parameters. In addition, when having someone repair and debug the machine, the maintenance personnel may use some temporary IP addresses. If they forget to change them, conflicts may occur. Another thing we hate is when someone steals someone else's IP address.

After receiving the conflict report, there are several things to do. First, determine the VLAN where the conflict occurs. Through the VLAN definition of IP planning and the conflicting IP address, you can find the network segment where the conflicting address is located. This is very critical to successfully find the MAC address of the network card, because some network commands cannot be accessed across network segments.

Let's first isolate the client from the network. For computers with illegal IP addresses, let's just let it run. This way we have a chance to find it. Then do some simple tests on the network. The main commands are the ping command and the arp command. Use the ping command, assuming that the conflicting IP address is 146.127.35.3. In the msdos window, the command format is as follows:

C:\WIDOWS\〉ping 146.127.35.3

Request timed out

Reply from 146.127.35.3: bytes=32 time〈1ms TTL=128〉

Reply from 146.127.35.3: bytes=32 time〈1ms TTL=128〉

Reply from 146.127.35.3: bytes=32 time〈1ms TTL=128〉

In fact, initially I planned to use "Start"-"Run" to execute the ping command (in actual operation, I am more accustomed to (for using this method), but it went too fast and I couldn't see it, so I had to turn to msdos for help.

The reason why we want to ping this machine has two purposes. First, I must make sure that the machine I am looking for is indeed on the network. Second, I want to know the MAC address of the network card of this machine. So how can we know its MAC address? This requires the use of the second command arp: the arp command can only be used in a certain VLAN. It is a low-layer protocol and cannot cross routes.

C:\WIDOWS\〉arp -a

Interface: ...... on Inerface ......

Internet Address/Physical Address/Type

146.127.35.3/00-00-22-35-62-53/ dynamic

This means that the MAC address of the network card at the conflicting IP address 146.127.35.3 is 00 -00-22-35-62-53. The next thing we are looking for is the specific physical location of the network card with the MAC address 00-00-22-35-62-53.

The network card of each client is directly connected to the second-level switch. Next, facing a large number of Ethernet switches, we need to find the switch port corresponding to the conflicting MAC.

Because the device connected to the customer in my network is Bay's 303/304, I will use 303 as an example to see how to find the port location of a certain MAC address. There are many methods for Bay303's network management. The following only uses the Web browser method to explain the method of finding illegal MAC.

Before searching, you must first find out the location of the switches in the VLAN and find out the IP addresses of these switches. You can use the switch addresses to access the network management information of the switches. We need to do the following work. First, start the browser on the network administrator's machine, type in the IP address of the switch, enter the user name and password after seeing the login information prompted, and then enter the "MAC Address Table" option, so that The following information can be obtained:

index/mac address/learned on port/learning method/filter

packets to this address

1 00:00:21 :34:63:56 13 dynamic no

2 00:00:81:65:c3:a0 n/a static no

3 00:00:a2:f7:c3 ：e4 25 dynamic no

4 00:00:22:35:62:53 2 dynamic no

Haha, you can see the 4th item of the index, which is exactly what we want Find the MAC address and its port number is 2. According to the comprehensive wiring data, we can accurately find out the physical location of the corresponding information point, and further we can locate the location of the connected microcomputer. Of course, you may not be so convenient when solving this problem. You often need to search many switches. You may feel a "relief" when you finally find the MAC address. When there are a large number of switches in a VLAN, we need to search these switches one by one.

If there is a downstream switch in the port of a switch, in this case, because the switch supports multiple MAC addresses, it will save the record of the lower-level MAC in the upper-level MAC table, so you must first search for the upper-level switch. MAC table, determine the more specific location and then search for the next-level switch, which can greatly narrow the search scope.

In the local area network, we often encounter this kind of IP address conflict problem, especially when the user scale is very large, it will make the search work more difficult, so the network administrator must be careful about this. Only by taking action can the final solution be achieved. There are currently two solutions, one is to use dynamic IP address allocation (DHCP), and the other is to use static address allocation, but the management of MAC addresses must be strengthened.

The biggest advantage of using dynamic IP address allocation (DHCP) is that the configuration of the client network is very simple, so that we can set up the network connection ourselves even without the help and intervention of the administrator. However, because the IP address is dynamically assigned, the network administrator cannot identify the customer from the IP address, and the corresponding IP layer management will not be effective. In addition, it has a drawback. Using dynamic IP address allocation requires setting up an additional DHCP server.

If static IP address allocation is used, we can first carry out reasonable IP address planning for each department, so that we can easily track and manage on the third layer. At the same time, if we strengthen the management of MAC addresses , can effectively solve this problem.