Analysis of Abnormal routing caused by improper configuration of local IP addresses

We all know that you must be careful when configuring routes on routers and layer-3 switches. There must be no error in IP addresses and subnet masks. Recently, when I conduct a network debugging, I found that improper configuration of the local IP address also causes route exceptions.

Fault description

My company often needs to debug the network. Recently, the company released a website that requires access debugging. I used my laptop to debug and modify the IP address 211.153.80.29 in the company server group. the IP address of the accessed website is 211.153.80.1 and the domain name is "www.ftedu.gov.cn ". After debugging, the website can be accessed normally in the Organization. When I got home, I tried to access through ADSL. Because ADSL uses the PPPoE dial-up method, you can access the Internet without having to modify the IP address of the laptop you set up in the company. There is no problem accessing SOHU or Sina, QQ and MSN can also log on normally, but they cannot access the company's 211.153.80.1 server. The message "this page cannot be displayed" appears. The author uses the domain name for access, and the fault still exists.

Troubleshooting

In order to find out the root cause of the problem, I immediately called my colleagues and asked them to visit the test website through ADSL at home. As a result, both the IP address and domain name can be used for normal access. Therefore, the author locates the root cause of the fault on the Local Computer and starts troubleshooting.

Step 1: because the company uses a firewall, PingIP cannot obtain feedback, so I use the TRACERT command. After "tracert 211.153.80.1" is entered in the Command Prompt window, "Request timed out" is displayed immediately ". This indicates that the packet sent to 211.153.80.1 does not reach the first routing device.

Step 2: Continue to check if DNS is faulty. In command line mode, enter "nslookup", enter "www.ftedu.gov.cn", and press enter to find that the server can correctly parse the IP address "211.153.80.1 ", the problem is not related to DNS.

Step 3: try to track the neighboring IP address, that is, enter "tracert 211.153.81.1" in command line mode. The result shows that data packets can pass through five routing devices based on the obtained route information.

Step 4: after entering the IPCONFIG command, I found that the IP address of the local computer is still in the company's 211.153.80.29, And the subnet mask is 255.255.255.0. I suspect that this setting causes a fault and will immediately select the IP address as automatically obtained. After dialing the Internet again, the fault can be solved, and the server of the Organization can be normally Accessed at home.

Fault Analysis

I reset the IP address of the computer to 211.153.80.29. In command line mode, enter "route print" to query the local route to find the root cause of the fault (see figure ). Because the IP address of the Local Computer is set to 211.153.80.29, When you access the CIDR Block 211.153.80.0, the system sends the data packet directly to 211.153.80.29 instead of the default gateway address 61.51.199.192, as a result, the correct route information cannot be found.

Summary

Through this fault, we can gain the experience that we should pay attention to the setting of local IP addresses. If improper IP addresses cause local route exceptions, some network segments cannot be accessed normally. We recommend that you cancel the use of unused IP addresses in the case of such faults.