Normal IP packets consist of the IP header and the IP data, or payload. The IP header contains information about where the packet came from, where it should be going to, what kind of (sub-) protocol the packet has, the size of the packet, the time-to-live (TTL, sometimes called hopcount), some option bits that tweak little things, and finally an extra verification number, called the checksum. The checksum is a simple addition of all numbers in the IP packet. If one number gets accidentally changed during transmission, the checksum will be different, allowing the packet to be recognized as 'broken'.
For instance, this packet could be a UDP packet, in which case the protocol field in the IP header would have the value 17. It has a source and destination IP address, and a source port and destination port, since the UDP protocol uses ports. Within the header, there is a checksum that can be used by routers to see if the packet has been mangled. If so, the...