This means communicating with a device on a different network. As discussed previously, an IP address consists of 2 parts. The 1st part indicative of the network which the IP address belongs and the 2nd indicative of the host. When a packet is sent from one network to another, the IP address will clearly show that they are of different networks. Example: IP address of the sending device (source IP address) – 192.168.1.110 (PC1) IP address of the receiving device (destination IP address) – 172.16.1.99 (Web Server) Whenever a message (packet) is being sent from one network to another network, the packet cannot go directly to the destination device because it is in a different network from the sender. The message leaves the source network through the default gateway of the source network - the final exit from the source network and usually on a router. It then travels through and received by other routers or the router at the entrance into the destination network. This router at the entrance of the destination network will have the default gateway into the destination network. The default gateway of the destination network is the entrance into that network. This last router de-encapsulates the packet translating it into a frame (as discussed previously). The network address and the Mac address come into play when sending packets across different networks. Each device will know the IP address of its default gateway among other settings. The default gateway address of the source device is the IP address of the router interface connected to the same local network as the source device while the default gateway address of the destination device is the address of the router interface connected to the same local network as the destination device device. The MAC address of the sending device (source) in this example, PC1 is AA-AA-AA-AA-AA-AA. However the destination MAC address when the receiving device is on a different network from the source will be the MAC address of the default gateway or router. For this purpose, the MAC address of the R1 Ethernet interface – 11-11-11-11-11-11, which is connected to the PC1 network is the destination MAC address. The packet is received on R1 from the source device. R1 sends the packet to its destination which is the Web Server. However for this particular example, there are a couple of routers between R1 and the destination device. Once the IP address of the default gateway is known, ARP (as discussed in the last topic) will come into play, to establish the MAC address of that default gateway which is then added to the frame. References http://paf.dias.ac.cy/main/files/cisco_rs_ite/CCNA-R-S_ITN/course/module3/3.3.3.2/3.3.3.2.html |
CategoriesAuthor I have a passion for networking and use this medium for tutorials in Networking Archives
April 2017
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