To learn more on how the frame is created, visit the Data Encapsulation - Decapsulation.ħ) The Datalink Layer of Host A hands the frame to the Physical layer which encodes the 1s and 0s into a digital signal and transmits this out on the local physical network.Ĩ)The signal is picked up by the router's E0 interface and reads the frame. At the end of the frame, in the FCS portion of the frame, the Datalink Layer will stick a Cyclic Redundancy Check (CRC) to make sure the receiving machine (the router) can figure out if the frame it received has been corrupted. This includes the source and destination hardware address (MAC) and the type field which specifies the Network Layer protocol e.g IPv4 (that's the IP version we use), ARP. This packet includes the source and destination IP address as well as the ICMP echo request which was specified in the Network Layer.Ħ) The Datalink Layer of Host A creates a frame, which encapsulates the packet with the information needed to transmit on the local network.
Now, the Network Layer hands down to the Datalink Layer the packet it generated with the ping (ICMP echo request), along with the hardware address of the router. Host A now has everything it needs in order to transmit a packet out on the local network to the router. Because it takes some time for an ARP to be sent and the requested machine to respond with its MAC address, by the time all that happens, the TTL (Time To Live) of the first ping packet has expired, so it times out!ĥ) The router responds with the hardware address of its E0 interface, to which the 192.168.0.1 IP is bound. This is also one of the reasons why sometimes the first "ping" will timeout. The router identifies that IP address as its own and must answer, so it sends back to Host A a reply, giving it the MAC address of its E0 interface. This broadcast contains the following "What is the hardware (MAC) address for IP 192.168.0.1 ? ". To get the hardware address, Host A looks in its ARP cache - a memory location where these MAC addresses are stored for a few seconds.Ĥ) If it doesn't find it in there it means that either a long time has passed since it last contacted the router or it simply hasn't resolved the IP address of the router (192.168.0.1) to a hardware address (MAC). Since this is a request for a remote host, which means it is not destined to be sent to a host on the local network, the packet must be sent to the router (the gateway for Network A) so that it can be routed to the correct remote network (which is Network B).ģ) Now, for Host A to send the packet to the router, it needs to know the hardware address of the router's interface which is connected to its network (Network A), in case you didn't realise, we are talking about the MAC (Media Access Control) address of interface E0. Now, we are going to describe step by step what happens when Host A (Network A) wants to communicate with Host B (Network B) which is on a different network.ġ) Host A opens a command prompt and enters >Ping 200.200.200.5.Ģ) IP works with the Address Resolution Protocol (ARP) to determine which network this packet is destined for by looking at the IP address and the subnet mask of Host A. These interfaces are just like the interface on your network card (RJ-45), but built into the router. Both networks are connected via a router (Router A) which has 2 interfaces: E0 and E1. In our example, we have 2 networks, Network A and Network B. The process we are going to explain is fairly simple and doesn't really change, no matter how big your network is. You actually need a bit more information than just the IP Address ! When I was new to the networking area, I thought that all you needed was the IP Address of the machine you wanted to contact but so little did I know. We are going to analyse what happens when routing occurs on a network ( IP routing process).
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