controlled, and which device can transmit at which point.
Switching and switches, as described later in this
chapter, operate at the network access layer.
The Internet layer of the TCP/IP reference model
corresponds to the network layer of the OSI model in
terms of functions and characteristics. It is responsible
for transmitting data across disparate and distant
networks in a process called data routing. Routers are
devices that operate at the Internet layer and perform the
routing function. As you might have guessed from the
name of the reference model, Internet Protocol (IP) is
the main protocol operating at the Internet layer. The IP
protocol performs two basic functions:
Device addressing and identification
Data packet routingDevices connected to a TCP/IP network are identified by
their IP addresses. Two addressing solutions are
currently supported on the Internet: IP version 4 (IPv4)
and IP version 6 (IPv6). IPv4 uses addressing based on
32 bits, which means that approximately 4 billion devices
can be uniquely identified on the network. As you can
imagine, there are more than 4 billion endpoints
connected to the Internet today. Temporary solutions to
address the lack of IPv4 addresses have been developed
over the years, including private IP address subnets and
Network Address Translation (NAT), both of which are
discussed later in this chapter. The permanent fix to this
problem was the development and standardization in
1998 of IPv6, which uses 128-bit addresses and is able to
provide unique addresses for a gigantic number of
endpoints and devices. Both IPv4- and IPv6-addressed
devices are currently supported and connected to the
Internet, but IPv4 is slowly being phased out.
The second function of the Internet layer is data packet
routing. Routing packets means forwarding data from its
source toward the destination through a network of