Introduction
4
The Complete IP Connection
The following are the steps that a frame of data will take when being transmitted from an originating station on an IP network
to a destination station on a different IP network. In this example, the two networks are separated by a third network.
•
Originating station will send an ARP request if it does not have the MAC address of the destination station.
•
Local router will see ARP request and send an ARP reply to the originating station with the MAC address of the local
router port.
•
Originating station will send the data frame addressed to the IP address of the destination station and the MAC
address of the local router port.
•
Local router will receive the data frame and strip off the MAC portion. The resulting IP frame will be examined to
determine the destination IP address.
•
Local router will look in its routing table to find the IP address of the router to send the IP frame to next. The local
router will see that the destination router is the next router.
•
Local router will look in its ARP cache to find the MAC address of the destination router as determined by the IP
address in the routing table.
•
Local router will rebuild the complete frame with a new MAC header indicating the MAC address of the destination
router. Remember that the local router does not alter the destination IP address, so the destination IP address will
still be the IP address of the destination station.
•
Destination router will receive the data frame and strip off the MAC portion. The resulting IP frame will be
examined to determine the destination IP address.
•
Destination router will look in its routing table to find the IP address of the router to send the IP frame to next. The
destination router will see that the destination IP address is on a locally connected network.
•
Destination router will look in its ARP cache to see if it has a MAC address for the destination IP address. If it does
not have an entry, the destination router will generate an ARP request. The destination station will send an ARP
reply.
•
Destination router will rebuild the complete frame with a new MAC header indicating the MAC address of the
destination station. The destination IP address once again will be unchanged and remain as the destination station IP
address.
•
Destination station will receive the data frame and process it.
If the destination station wishes to send a frame back to the originating station, the process will happen in the reverse
direction.
If the path from the originating station to the destination station causes the frame to pass through more than two routers, the
above process will simply be extended to include the interaction between the intermediate routers.
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