A packet is the unit of data that is routed between an origin and a destination on the Internet or any other packet-switched network. Network packets are small (around 1.5 KBS for Ethernet packets and 64 KBS for IP packet payloads) amounts of data passed over TCP/IP networks. As an example, e-mails and web pages will make use of network packets to send information back and forth to the user and recipients. The goal of a network packet is to send information reliably so data does not have to be sent as a single, large file.
Each packet sent includes information such as the source and destination, the protocols or identifications.
How network packets work
When any file (such as an e-mail message, HTML file, GIF file or URL request) is sent from one place to another on the Internet, the Transmission Control Protocol (TCP) layer of TCP/IP divides the file into "chunks" of an efficient size for routing. Each of these packets is separately numbered and includes the Internet address of the destination. The individual packets for a given file may travel different routes through the Internet. When they have all arrived, they are reassembled into the original file (by the TCP layer at the receiving end).
A packet-switching scheme is an efficient way to handle transmissions on a connection-less network such as the Internet. An alternative scheme, circuit-switched, is used for networks allocated for voice connections. In circuit-switching, lines in the network are shared among many users as with packet-switching, but each connection requires the dedication of a particular path for the duration of the connection.
Parts of a network packet
Network packets are made up of three different parts, the header, the payload and the trailer. Network packets can be thought of conceptually to postal packages. The header is the box/envelope, the payload is the box/envelope’s content, and the trailer is the signature.
Broken down even further, the header may consist of the following pieces:
- The internet protocol.
- The size of the header and payload.
- The source and destination address.
- A sixteen-bit identification number.
- Flags, which are used to let a router know if it can fragment a packet.
- Fragmentation offsets, which reconstructs fragmented packets.
- The time to live (TTL).
- A checksum, which detects errors.
- The number of hops a packet can make.
A payload is the actual data being sent. Trailers, also called the footer, are the bits which signify the end of a packet.