As a packet travels through the levels of the TCP/IP protocol, each layer adds it’s own header information to the datagram.
This process of each layer encoding it’s own management information into the existing datagram is called nesting , or encapsulation .
Various headers contain the source and destination address, checksum, protocol specific information, and other handling instructions.
At the receiving end the headers are stripped off by the appropriate level, the header contents are examined for special handling information, and the datagram is delivered to the appropriate application.
We are running out of addresses under the current (IPv4) addressing scheme.
If every class A, Class B, and Class C network address was in use using classful addresses, there would be ((127 * 16,000,000) + (16384 * 65,000) + (2,097,152 * 254)) (or 3,629,636,608) hosts on the Internet. (3.6 gigahosts)
The remainder of the addresses are the “zero”, and “broadcast hosts (overhead).
If subnetworking is in use, even more of the address space is lost to “overhead”.
Real Soon Now a new version of IP will be released. This version is known as IPV6 (Internet Protocol version 6).