to your friend in
Boston with one 29 cent stamp.
A letter that you might send a friend contains 3 pieces of information. There
is the address it is being sent to, the return address, and the letter itself
which is just a bunch of information. By convention, the destination and
the return address appear on the outside of the letter, so the post office
workers can find it easily. The only service they provide is moving the
letter to its destination (if it has a stamp). There is also a limitation
on the weight of a letter, so you couldn't send the OED
to your friend in
Boston with one 29 cent stamp.
In the Nachos network, a packet will contain the source machine and box number, the destination machine and box number, and some number of bytes of packet data. The network itself just moves the packet, all of the other structure is convention. These conventions on the information content and structure of this information in a packet are called protocols.