Practical Industrial Data Networks:Design, Installation and ...

274 Practical Industrial Data Networks: Design, Installation and TroubleshootingIP address over the Internet, the message will not be transported across the Internet andhence the desired network cannot be reached.16.4.4.5 Duplicate IP addressesSince an IP address is the Internet equivalent of a postal address, it is obvious thatduplicate IP addresses cannot be tolerated. If a host is booted up, it tries to establish if anyother host with the same IP address is available on the local network. If this is found to betrue, the booting up machine will not proceed with logging on to the network and bothmachines with the duplicate IP address will display error messages in this regard.16.4.4.6 Incorrect network ID – different netIDs on the same physical networkAs explained in the chapter on TCP/IP, an IP address consists of two parts, namely, anetwork ID (netID), which is the equivalent of a postal zip code and a hostID, which isthe equivalent of a street address. If two machines on the same network have differentnetIDs, their ‘zip codes’ will differ and hence the system will not recognize them ascoexisting on the same network. Even if they are physically connected to the sameEthernet network, they will not be able to communicate directly with each other usingTCP/IP.16.4.4.7 Incorrect subnet maskAs explained in the chapter on TCP/IP, the subnet mask indicates the boundary betweenthe netID and the hostID. A faulty subnet mask, when applied to an IP address, couldresult in a netID (zip code) that includes bits from the adjacent hostID and hence looksdifferent than the netID of the machine wishing to send a message. The sending host willtherefore erroneously believe that the destination host exists on another network and thatthe packets have to be forwarded to the local router for delivery to the remote network.If the local router is not present (no default gateway specified) the sender will give upand not even try to deliver the packet. If, on the other hand, a router is present (defaultgateway specified), the sender will deliver the packet to the router. The router will thenrealize that there is nothing to forward since the recipient does in fact, live on the localnetwork, and it will try to deliver the packet to the intended recipient. Although thepacket eventually gets delivered, it leads to a lot of unnecessary packets transmitted aswell as unnecessary time delays.16.4.4.8 Incorrect or absent default gateway(s)An incorrect or absent default gateway in the TCP configuration screen means that a host,wishing to send a message to another host on a different network, is not able to do so. Thefollowing is an example:Assuming that a host with IP address 192.168.0.1 wishes to ping a non-existent host onIP address 192.168.0.2. The subnet mask is 255.255.255.0. The pinging host applies themask to both IP addresses and comes up with a result of 192.168.0.0 in both cases.Realizing that the destination host resides on the same network as the sender, it proceedsto ping 192.168.0.2. Obviously, there will be no response from the missing machine andhence a time-out will occur. The sending host will issue a time-out message in this regard.Now consider a scenario where the destination host is 193.168.0.2 and there is no validdefault gateway entry. After applying the subnet mask, the sending host realizes that thedestination resides on another network and that it therefore needs a valid default gateway.