The Illustrated Network- P13:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 3 Network Link Technologies 89 DIX Ethernet Frame Structure Destination Address Source Address Type Information FCS 6 bytes 6 bytes 2 bytes 46-1500 bytes 4 bytes Type 0x0800 for IP packets IEEE LANs Frame Structure Destination Address Source Address Length Information FCS 6 bytes 6 bytes 2 bytes 48-1492 bytes 4 bytes 8 bytes of added overhead Logical Link Control LLC Destination Service Access Point DSAP 0XAA SNAP SAP Source Service Access Point SSAP 0XAA Control 0X03 same as in PPP Subnetwork Access Protocol SNAP Organizationally Unique ID X 0000 0000 usually Type 0X0800 for IPv4 packets 0X08DD for IPv6 etc. FIGURE Types of Ethernet frames. The frames for Gigabit and 10 Gigabit Ethernet differ in detail but follow the same general structure. MAC Addresses The MAC addresses used in 802 LAN frames are all 48 bits 6 bytes long. The first 24 bits 3 bytes are assigned by the IEEE to the manufacturer of the NIC manufacturers pay for them . This is the Organizationally Unique Identifier OUI . The last 24 bits 3 bytes are the NIC manufacturer s serial number for that NIC. Some protocol analyzers know the manufacturer s ID which is not public but seldom suppressed and display this along with the address. This is how Ethereal displays MAC addresses not only in hex but starting with Intel_ or Juniper_. Note that both frame types use the same familiar source and destination MAC address and use a 32-bit 4-byte frame check sequence FCS for frame-level error detection. The FCS used in both cases is a standard 32-bit cyclical redundancy check CRC-32 . The important difference is that the DIX Ethernet frame indicates information type frame content with a 2-byte type field 0x0800 means there is an IPv4 packet inside and 0x86DD means there is an IPv6 packet inside and the IEEE . CSMA CD frame places this Ethertype field at the end of an additional 8 bytes of overhead called the Subnetwork Access Protocol SNAP header. Another 3 bytes are the OUI given to the .
