1.5 Addressing and Packets

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In the early store-and-forward networks it was important to know the source and destination computers for every message. Each computer was given a unique name or number that was called the “address” of the computer. To send a message to another computer, you needed to add the source and destination address to the message before sending the message along its way. By having a source and destination address in each message, the computers that stored and forwarded the message would be able to pick the best path for the message if more than one path was available.
When a long message was split into much smaller packets and each packet was sent individually, the source and destination addresses had to be added to each packet, so that routers could choose the best path to forward each packet of the message. In addition to the source and destination addresses, it was also necessary to add data to each packet indicating the “offset” or position of the packet in the overall message so that the receiving computer could put the packets back together in the right order to reconstruct the original message.

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