MAC addresses and their functionality - Homenet Howto
At the destination, the frame is stripped and the destination IP address is verified. IP will add a header consisting of a source address of By the subnet mask of Host3, it is determined that the destination lies in a remote network. Hence the frame will need the MAC address of the default gateway as destination. Router3 strip the frame header and look at the destination IP address in the IP header.
From its routing table, it will know that the packet needs to go to Router2. Router2 receives the frame and repeats process to send the packet to Router 1. Router1 receives the frame from Router2 and removes the frame. By the destination IP address it knows that the packet belongs to a directly connected interface. When Host1 receives the frame, it verifies the destination address, strips the frame and IP header and sends the TCP segment to layer 4.
Exam Alert : Remember that the source and destination IP address do not change throughout the process while the source and destination MAC address changes at each segment. First the local machine will compare the destination IP address, since its outside of our network, it does not see the MAC address of the destination host. So our packet now looks at the routing table, it doesn't know the address of the destination IP, so it sends it out to the default gateway another router.
Remember MAC addresses are only reached through the same network. So what does it do? The router looks at the packet and confirms the destination MAC address, but it's not the final destination IP address, so it keeps looking at the routing table to forward the packet to another IP address that can help the packet move along to its destination.
Everytime the packet moves, it strips the old source and destination MAC address and updates the packet with the new source and destination MAC addresses. Once the packet gets forwarded to the same network, we use ARP to find the final destination MAC address During this process, our packet doesn't change the source or destination IP address. IP will send the IP packets to layer-2, e.
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Not all layer-2 protocols use MAC addresses. Other layer-2 protocols use something else, or nothing at all for addressing. Ethernet will encapsulate the layer-3 IP packets in layer-2 ethernet frames. The ethernet frames will have the source and destination MAC addresses. The host your computer will mask both the source and destination layer-3 IP addresses to determine if the destination address is on the same LAN.
If the destination is on the same LAN, the frames will be addressed with the MAC address of the destination host, otherwise they are addressed with the MAC address of the configured gateway as the destination host. This will require resolving the layer-3 addresses to the layer-2 addresses.
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Layer-2 will then send the frames to layer-1 physical interface to be encoded and placed on the wire. When the frame is received by the router, the layer-2 frame, including the MAC addresses, is stripped off the packet and discarded, losing the original MAC addresses. The router will decrement the TTL and create a new header checksum, and inspect the layer-3 IP packet for the destination address.
The router will then look up how to get to the destination IP address in its routing table.
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If it doesn't find a route to the destination IP address in its routing table, the packet is discarded, otherwise the router will switch the packet to the next interface toward the destination and create a new layer-2 frame for the new interface. The next router in the path will get the new frame, and it will repeat the routing process.
This happens until the packet gets to the last router, and the last router will put on a frame for the destination LAN. The destination host will process in reverse order from the sending host. The destination host will have the source layer-3 and layer-4 addresses from the packet and segment, and it can send back to the source using those addresses in the same manner that the original host was the sender. Routers are layer-3 devices, and they only look at the layer-3 packet, not into the layer-4 segment.
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Every router has a default gateway for a network range, so if it handles your packet going from your IP A to some remote IP C it does not need to rewrite anything. It just knows, that it has to forward this packet to the interface, say, 3.
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Of course it may have route cost and many other settings, but generally it is like that. It does not need to remember and store your table. It has only route table to intercommunicate with neighbour routers. At 0 value the packet will be killed. L2 devices change nothing. Generic router does not change neither IP nor port souce and destination values.