Asymmetric network subnet mask causes network impassability is "the culprit" _ Networking Tutorials

The network does not pass, this is the most network problem table image, but if you can access me, but I can not access you, this problem is more troublesome. This article is the author in the actual combat in a subnet mask caused by the fault summary.

The network is a phenomenon that often occurs, encounter this phenomenon, I believe many people will focus on network cable, NIC driver, network card parameters, while checking the network card parameters, they may focus on the IP address, gateway address, DNS server or DHCP server and other key parameters. In fact, there is also a network parameter-subnet mask, also very important, if we ignore the setting of it or set it wrong, also can cause network failure. Because the subnet mask parameter is usually not people pay attention to, once the parameter caused by network failure, then the troubleshooting is often easy to make detours!

Failure phenomenon: Asymmetric network

A corporation and its subsidiaries are directly connected to the local telecommunications sector through broadband optical fiber and comprise the company's internal network through a telecommunications network; at ordinary times, each subsidiary network not only can access each other, but also can access the corporate network, and even access to Internet networks, while the head office can access the network of subsidiaries , and access to Internet networks. Recently, a subsidiary reaction, unable to visit the head office of the Web server, so that they can not through the network to report data to the head office. Head office network administrator after the initial inspection, found that the company and a subsidiary of the network Ping, but not only the head office to ping the network of other subsidiaries, but also access to Internet networks, a subsidiary, although unable to ping other subsidiaries of the network and corporate network, But it has access to Internet networks.

Troubleshooting: Firewall, Telecommunications aspects? Subnet Mask!

Through the above fault description, the head office network administrator thinks that since the head office can ping other subsidiaries network, which indicates that the company's network working status is normal, the problem is likely to occur in a subsidiary network or local telecommunications department, a subsidiary network can access Internet network, This shows that a subsidiary network and the local telecommunications network in the physical connection should have no problems, the network administrator also initially believed that a subsidiary network in the parameters of the configuration of no problem.

However, a subsidiary network could not ping other affiliate networks, indicating that the cause of the failure should be in a subsidiary network or local telecommunications department. However, the network administrator is very puzzled: Usually, the head office network and a subsidiary network, as long as the Internet network can access the same site content, they should be able to access each other, unless they have enabled firewall filtering to deny access to their respective networks. However, after the actual inspection, both sides of the network administrator has temporarily shut down the firewall, or can not solve the network failure, which means that the network failure and firewall filtering function.

After excluding the firewall factor, the head office's network administrator believes that the cause of the failure may appear in the local telecommunications department to the a subsidiary network routing settings, so the network administrator logged into the local router's background management interface, and in this interface to track a subsidiary network routing. To find out, the network administrator tracked the routing of several other subsidiary networks, and after careful comparison of the tracking results, The network administrator found that there is a clear loop between the 202.102.11.156 address and the 202.102.11.254 address, find the address record information, found that these addresses are from the telecommunications department there, it seems the crux of the problem here!

Considering the cause of the failure involved in the telecommunications sector, the administrator is very cautious, because the telecommunications sector if the phenomenon of address loops, then believe that many users will very early to the telecommunications Department for help, do not solve the problem now? So the network administrator decides to look up the problem from itself, If there's no reason to find out, it is also not too late to repair the network fault to the telecommunication department. After taking the notice, the network administrator inspected the head office of the server, the router, and the other subsidiaries of the network administrators to carry out the test, after repeated testing, the network administrator confirmed that the network does not have any problems.

In the formal repair of network failures to the telecommunications department, the head office network administrator has not forgotten to tell a subsidiary of the network administrator, to his local subnet parameter settings also carry out a careful examination, and asked him to timely feedback inspection results, so that the company decided whether unified to the Telecommunications department repair network failure. Not long, a subsidiary of the network administrator called to say, when checking the local routing settings, found that when setting the subnet mask parameters, "255.255.255.192" was set to "255.255.255.0"; According to a subsidiary's network administrator, Although he is to the head office of the allocation of the parameter table set, but because of the operation of the habit of accidentally the router subnet mask parameters into the intranet mask address. After the address has been modified, a subsidiary's network will immediately be able to access the head Office server. At this point, this article mentioned the network failure, has been successfully resolved, and the failure of the final "culprit" unexpectedly is not noticeable subnet mask.

Failure Summary: Why can the internet, but not even headquarters

After finding the cause of the failure, it is not difficult to explain some of the strange network phenomena mentioned earlier. For example, a subsidiary's network can still access an Internet network if the subnet mask parameter is improperly set, because the router for that subnet has a default route record that can forward the network data that is not a subsidiary to the extranet port. So a subsidiary is able to access content in Internet networks.

A subsidiary's network is unable to ping the company's network, mainly because of the subsidiary's subnet mask parameter settings caused by error. We know that the host before sending the Internet request, often will first determine whether the target host is in the local subnet, if the same subnet will directly send the Internet request to the target host, if not in the same subnet, will send the Internet request to the designated gateway. So how does the local host recognize that the target host belongs to the local subnet or extranet? This is mainly dependent on the subnet mask address. Suppose, in normal state, a subsidiary's network mask address is 255.255.255.192, at which point a subsidiary's 202.102.11.87 host tries to access the head Office's Web server (the server address is assumed to be 202.102.11.2), The landlord's opportunity to think that the 202.102.11.2 server is in the extranet, so it will send the Internet request to the local gateway, at this time a subsidiary of the 202.102.11.87 host can ping the head office of the server. Once the network mask address of a subsidiary becomes "255.255.255.0", The local host will assume that the 202.102.11.87 address and 202.102.11.2 address are in the same network segment, so the local host will send the Internet request directly to the target host, rather than forwarding it to the local gateway, in fact, there is no IP address in the local subnet to 202.102.11.2 this target host, so a The subsidiary's 202.102.11.87 host will not be able to ping the head office's server. The main office of the network can not ping a subsidiary of the network, mainly because of a subsidiary of the network mask address error, resulting from the extranet data request information can not correctly reach the target host, it will be considered a subsidiary of the routing.


Communication is a common phenomenon, encounter this phenomenon, I believe many people will focus on network cable, NIC driver, network card parameters, while checking the network card parameters, they may focus on the IP address, gateway address, DNS server or DHCP server and other key parameters. In fact, there is also a network parameter--Subnet mask ...


Small hint: the important role of subnet mask

Many friends who first contact the network often think that only by looking at the IP address can determine whether two IP addresses belong to the same subnet, in fact they ignore the subnet mask this important parameter. In general, observe whether two IP addresses are in the same subnet, looking at both the IP address and the subnet mask address. Say The IP address of a workstation is 10.192.0.1, the corresponding subnet mask address is 255.255.255.0, the other workstation's IP address is 10.192.1.1, and the corresponding subnet mask address is 255.255.255.0, then they are not in the same subnet, because through the subnet mask address we You can see that the previous workstation has a subnet network number of 10.192.0.0, and the next workstation has a subnet network number of 10.192.1.0.

Let's say, The IP address of a workstation is 10.192.0.1, the corresponding subnet mask address is 255.255.0.0, the other workstation's IP address is 10.192.1.1, and the corresponding subnet mask address is 255.255.0.0, then they are in the same subnet, because we can see the front by the subnet mask address A workstation has a subnet network number of 10.192.0.0, and the next workstation has a subnet network number of 10.192.0.0.

So, with the above two examples, we can see that the same two IP addresses, if their corresponding subnet mask address is different, then these two addresses may belong to different subnets, it may be in the same subnet.