Page 745 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

Data Link Layer

The Data Link layer (layer 2) is responsible for formatting the packet
from the Network layer into the proper format for transmission. The

proper format is determined by the hardware and the technology of
the network. There are numerous possibilities, such as Ethernet (IEEE
802.3), Token Ring (IEEE 802.5), asynchronous transfer mode
(ATM), Fiber Distributed Data Interface (FDDI), and Copper DDI
(CDDI). However, only Ethernet remains a common Data Link layer

technology in use in modern networks. Within the Data Link layer
resides the technology-specific protocols that convert the packet into a
properly formatted frame. Once the frame is formatted, it is sent to the
Physical layer for transmission.

The following list includes some of the protocols found within the Data
Link layer:

Serial Line Internet Protocol (SLIP)

Point-to-Point Protocol (PPP)

Address Resolution Protocol (ARP)

Layer 2 Forwarding (L2F)

Layer 2 Tunneling Protocol (L2TP)

Point-to-Point Tunneling Protocol (PPTP)

Integrated Services Digital Network (ISDN)

Part of the processing performed on the data within the Data Link
layer includes adding the hardware source and destination addresses
to the frame. The hardware address is the Media Access Control
(MAC) address, which is a 6-byte (48-bit) binary address written in
hexadecimal notation (for example, 00-13-02-1F-58-F5). The first 3

bytes (24 bits) of the address denote the vendor or manufacturer of the
physical network interface. This is known as the Organizationally
Unique Identifier (OUI). OUIs are registered with the Institute of
Electrical and Electronics Engineers (IEEE), which controls their
issuance. The OUI can be used to discover the manufacturer of a NIC

through the IEEE website at
http://standards.ieee.org/regauth/oui/index.shtml. The last 3 bytes