The initial portion of a packet or a frame. The header contains control information such as addressing, routing, and protocol version. The format of this information depends on the protocol being used. For example, an Internet Protocol (IP) header contains information about the version of the IP protocol, the length of the header, the type of service used, the packet’s Time to Live (TTL), the source and destination address, and so on. Headers are used to control the flow of packets through the network or over the communication link.
The end of a frame sometimes has a smaller structure called a footer or trailer, but this usually contains only error-checking information. Control information is always placed in the header because this is the first portion of the packet or frame that is read by a networking device such as a switch or a router.