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IP Packet Delivery on a Local Area Network is a fundamental concept that all system and network administrators should have a clear understanding of. How do computers decide where to send the packets to? Should they send them directly to the target computers, the gateway, or both? The answer is simple. Computers use their local ROUTING table to make that determination. Yes, even computers have a routing table. Actually, if a network host is using TCP/IP to communicate on the network, it will have a routing table. Here is an example of a routing table taken from a computer with an IP address of  If you read through this routing table, you will
notice that if a packet’s network destination address is anything other than an address defined in the At first, these tables may be difficult to interpret. However, once you understand how to read it, it should be simple to determine how the computer will treat outgoing TCP/IP packets. Another thing you may note is that the table is relatively small. How can this computer communicate with basically any other computer on the planet with 9 entries in its routing table? Again, the answer is simple. If the target computer is not located on the local subnet, it simply sends the packet to the default gateway. It’s the router’s job to figure out what to do next. Let’s take a look at an example. As you notice in the next diagram, we have
depicted a network with two subnets segmented by a router. One subnet is defined on the So the next question you may have is how does the computer know if the target system is in the same subnet or on a different subnet. The host uses Boolean math, specifically the AND function. The system converts the source and destination IP to a binary number, applies the subnet mask to both and compares the results using the AND function. If the results are the same, the target must be in the same subnet. If the results are different, the target must be on a subnet. The AND function simply compares two numbers as follows. So what happens when WK1 (
192.168.1.1 192.168.1.2 The results indicate that both It uses the ARP protocol and sends out a broadcast on the local
subnet. Since this is a broadcast, the destination MAC address is Therefore if one of the hosts determines that its IP address matches that in the ARP packet, in this case, WK2 will respond to WK1 including its MAC address. Now that WK1 has WK2’s MAC, it can send the packet directly to WK2. The packet will include this information for delivery. Since the MAC is now targeted to WK2, WK2 will be the only system to bring the packet up the TCP/IP stack.
Let’s take another example. What will happen when WK1 wants to send a packet to WK3?
192.168.1.1 192.168.2.1 The results indicate that both It uses the ARP protocol and sends out a broadcast on the local subnet. Since this is a broadcast, the destination MAC address is Therefore if one of the hosts determines that its IP address matches that in the ARP packet, in this case, the router will respond to WK1 including its MAC address. Now that WK1 has the router’s MAC for the The packet will include this information for delivery. Note that the destination IP is that of WK3, but the destination MAC is that of the router.
When the router receives the packet, it will notice that the destination IP address is not of its own and belongs to another host. The router will use its local routing table to determine where to send the packet to. In this case, the router will find that the destination is connected to one of its interfaces. It will remove its MAC address from the packet and replace it with WK3’s once it obtains it via the ARP protocol. The packet will include this information for delivery. Note that the destination IP is still that of WK3, and now the destination MAC is that of WK3.
Once WK3 receives the packet, the entire cycle occurs in the same manner if WK3 needs to communicate back with WK1. That’s it. Pretty simple, huh? As you can see, it is very important to understand how the TCP/IP protocol suite works. When a router does not have destination network address in routing table?1) the title asks what a router would do if it received a packet and can not find the destination address in its routing table. If there is a default route then the router would forward the packet using the default route. If there is not a default then the router would return an unreachable to the source.
What would happen if a packet didn't make it to its destination?In a TCP connection between two hosts, each packet sent by the source host must be acknowledged by the destination host. When a packet is lost in transit, the destination host does not receive it and does not send the acknowledgment message for that packet.
What happens to a packet if the router does not find the network containing its IP address?If the network containing the IP address is not found, then the router sends the packet on a default route, usually up the backbone hierarchy to the next router. Hopefully the next router will know where to send the packet. If it does not, again the packet is routed upwards until it reaches a NSP backbone.
What happens if a router receives a packet but does not have an entry in its routing table for the destination network and does not have a default route configured?What does a router do when it receives a packet whose destination address is not listed in the routing table and the router does not have a default route configured? It discards the packet.
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If a router receive a packet without destination network address the packet will be
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