Network Access Layer

The Network Access layer is the lowest level of the TCP/IP protocol hierarchy. It is often ignored by users as it is well hidden by the better known mid-level protocols such as IP, TCP, and UDP, and higher level protocols such as SMTP, HTTP, and FTP. Functions performed at the network access layer include encapsulation of IP datagrams into frames to be transmitted by the network, and mapping IP addresses to physical hardware addresses. Much of the work that takes place at the network access layer is handled by software applications and drivers that are unique to individual pieces of hardware. Configuration often consists of simply selecting the appropriate driver for loading, and selecting TCP/IP as the protocol for use. Many computers come with this driver software pre-loaded and configured, or can automatically configure themselves via "plug-and-play" applications. A good example of configuration at the network access layer would be setting up a Windows NT system to use a 3-Com Etherlink III network interface card (NIC) with Ethernet. Under normal circumstances the NIC would be detected by the operating system at installation time. If this does not occur, or if the card is added at a later time, the installation procedure would consist mostly of installing the card and selecting the driver by choosing the manufacturer and model of the card from a list of available drivers. Some cards might additionally require minimal hardware configuration, such as selecting an I/O port, IRQ setting, and possibly whether to use the Twisted Pair or Co-Ax connector on the card. Once the correct driver is selected for the card and configured, the next step is to select TCP/IP from a list of available protocols to be used on the interface. Once this is done, the network access layer configuration is complete. Many configuration programs automatically move on to configuring the IP address, netmask, DNS servers, default gateways, and other parameters which actually relate to other layers and protocols. Some network access layer protocols do require extensive configuration. It is good to have an understanding of the more common of these and how they work. If a network manager will be dealing with remote access across serial communications lines, using POTS modems or ISDN lines for example, it is important to be familiar with serial point-to-point protocols such as SLIP and PPP. If these will be used on Network Access Server equipment, like modem stacks or terminal servers, it is valuable to be familiar with remote authentication protocols such as TACACS and RADIUS.