50 Networking Explained, Second Edition
identiﬁes the network to which a node is connected. In TCP/IP, this is called an Internet
address or an IP (for Internet Protocol) address. Network addresses operate at the network
layer. Each network node that is part of the Internet has a unique IP address. (Note: IP
addresses do more than simply identify the network. See Chapter 3 for additional informa-
tion about IP.) The third address is called the port address, which uniquely identiﬁes a spe-
ciﬁc user application such as e-mail. All network applications have corresponding
identiﬁers called port numbers. To send a message from one node to another, a message is
ﬁrst created at the application layer. It undergoes whatever formatting is required as it
descends the layers. When the message reaches the network layer, a network address is
assigned to the message. This network address identiﬁes the speciﬁc network to which the
destination host is connected. Depending on the protocol, this service is either connection-
less or connection-oriented. For example, TELNET and SMTP are connection-oriented
services. The network layer determines the path the message must follow to reach the des-
tination node. It also encapsulates packets into IP datagrams and passes them to the data
link layer. At the data link layer, the destination node’s hardware address is added to the
packet. This address uniquely identiﬁes the location of the destination node within the des-
tination network. The data link layer, among other tasks, also formats the packet into
frames, which are like packets but exist at a lower level and checks, the integrity of each
frame (see Chapter 5). Frames are then passed to the physical layer, which places them on
the medium for transmission.
47. OK. Enough of OSI. How does TCP/IP compare to OSI?
TCP/IP’s development preceded the OSI model by several years. Both had similar
design goals, however, to ﬁll a need for interoperability among heterogeneous computer
systems. Unlike OSI, TCP/IP was never intended to be an international standard. It was
developed to satisfy the need to interconnect various United States Department of Defense
projects, including computer networks, and to allow for the addition of dissimilar
machines to the networks in a systematic, standardized manner.
48. Does TCP/IP also have seven layers like OSI?
No. As a pre-OSI protocol architecture, it was not designed speciﬁcally with layers the
way the OSI model was designed and it does not ﬁt neatly into the seven layers of the OSI
model. However, we can envision TCP/IP’s layers as similar to the OSI layers since many
of TCP/IP’s functions are similar to those of the OSI model.
49. So what are the layers?
There is no universal agreement on the description of TCP/IP as a layered model. It is
frequently described as either a four- or ﬁve-layered model depending on an author’s per-
spective. For our purposes, we elect to describe TCP/IP as a four-layered architecture,
which is shown in Figure 2.14. Note that a ﬁve-layered TCP/IP model maintains OSI’s
physical and data link layers as separate levels instead of combining them into a single
layer as shown in Figure 2.14. In this scenario, TCP/IP’s ﬁrst layer is called the physical
layer and its second layer is called the network access layer.