It is more efficient and reliable to calculate a checksum or cyclic redundancy check over the contents of a packet than to check errors using character-by-character parity bit checking.
The packet trailer often contains error checking data to detect errors that occur during transmission.
Host addressing
Modern networks usually connect three or more host computers together; in such cases the packet header generally contains addressing information so that the packet is received by the correct host computer.
In complex networks constructed of multiple routing and switching nodes, like the ARPANET and the modern Internet, a series of packets sent from one host computer to another may follow different routes to reach the same destination. This technology is called packet switching.
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