Chapter 12: Networks Contents
Network Layer
Transport Layer
Application Layer
Transport layer
Network Access Layer
Data link layer
Physical layer
Host-to-host
Internet (IP)
Transport (TCP, UDP)
Application
(HTTP, FTP etc.)
ISO/OSI and TCP/IP Reference Model
models.
The various layers perform the following tasks:
❑ The host-to-network layer is responsible for transferring information from one computer to a
distant computer. It deals with the physical properties of the transmission medium^1 and with
dividing the data stream intoframesof a certain size to permit retransmission of data chunks
if transmission errors occur. If several computers are sharing a transmission line, the network
adapters must have a unique ID number known as aMAC addressthat is usually burned into the
hardware. An agreement between manufacturers ensures that this number is globally unique.
An example of a MAC address is08:00:46:2B:FE:E8.
In the view of the kernel, this layer is implemented by device drivers for network cards.
❑ The network layer of the OSI model is called theInternet layerin the TCP/IP model, but both
refer basically to the same task of exchanging data between any computers in a network, not
necessarily computers that are directly connected, as shown in Figure 12-2.
A direct transmission link between computers A and B is not possible because they are not phys-
ically connected to each other. The task of the network layer is therefore to find a route via which
the computers can talk to each other; for example, A–E–B or A–E–C–B.
A
C B
D E
Figure 12-2:
Network-linked
computers.
(^1) Predominantly coaxial cable, twisted-pair cable, and fiber opticlinks are used, but there is an increasing trend toward wireless
transmission.