A packet has two parts, like a parcel with a label.

• The payload is the actual data being carried, such as a slice of a photo.
• The header is the label: who sent it, where it is going and what number it is in the sequence.

Routers read the header to decide where to send the packet next. They do not need to look at the payload at all. A header typically holds fields such as:

• Source address: where the packet came from.
• Destination address: where it is going.
• Sequence number: which piece of the message this is (added by TCP, World 5).
• Time to live (TTL): a counter that stops lost packets wandering forever (World 2).
• Checksum: a small number for spotting damage (next lesson).