A packet is a little piece of data that is packaged for transmission over a packet switched network. Packets have a maximum size. If the message is larger the message is split up in a number of packages. to make the routing of packets possible in the network the packets are given a header. The information in the header varies with the protocol that is used for the transportation of the packets. The header always has to include the destination address. Besides that, it may also include information like the source address, a sequence number to reassemble the data stream in the correct order at the receiver and a checksum to guarantee the integrity of the received data. In some cases, a trailer is attached to identify the end of the packet. The trailer may also contain the information used to detect and correct errors.