1. Introduction to Ethernet In 1985, the Institute of Electrical and Electronics Engineers (IEEE) published standards for LANs. These standards start with the number 802. The standard for Ethernet is 802.3. Essentially, Ethernet and IEEE 802.3 are the same standards. In 1995, IEEE announced a standard for a 100-Mbps Ethernet. This was followed by standards for gigabit per second (Gbps, 1 billion bits per second) Ethernet in 1998 and 1999. In 2002 IEEE announced the standards for 10 Gigabit Ethernet All the standards are essentially compatible with the original Ethernet standard. An Ethernet frame could leave an older coax 10-Mbps NIC in a PC, be placed onto a 10-Gbps Ethernet fiber link, and end up at a 100-Mbps NIC.

2. Ethernet and the OSI Model Not Included

3. Ethernet and the OSI Model

4. MAC Rules and Collision Detection/Backoff

5. Ethernet

6. 10BaseT Half-Duplex Operation and Collision Domains Stations separated by hubs are within the same collision domain Let’s look at packet tracer to see how hubs work

7. Naming MAC addresses are sometimes referred to as burned-in addresses (BIA) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes.

8. Naming

9. Ethernet Encapsulation RFC 894

10. Ethernet Encapsulation Destination Address 6 Bytes Source Address 6 Bytes Type 2 Data 46-1500 Bytes CRC 4 Bytes Type 0800 2B IP Datagram 46-1500 Bytes Type 0806 2B ARP Request/Reply 28 Bytes PAD 18 Bytes

11. Ethernet Encapsulation 48-bit (6bytes) source and destination addresses. These are what we call hardware addresses. The Ethernet type field identifies the type of data that follows. The data field is the actual payload and must be at least 46 bytes If needed (frames with less than 46 bytes of payload), Pad bytes are inserted to assure that the frame is long enough. The CRC field is a cyclic Redundancy check (a checksum) that detects errors in the frame. (This is also called FCS or Frame Check Sequence)

12. Address Resolution Protocol: ARP Is needed in order to for the source to Learn the MAC address for the destination The IP address of the destination is used to learn the MAC address of the destination Two-step process: ARP Request and ARP Reply ARP Requests are sent to the broadcast address of FFFF.FFFF.FFFF and heard by all nodes in the same broadcast doamin (coming later) Only the node with the matching destination IP address replies AN ARP table is saved in RAM and can be viewed by using the command arp –a from DOS

13. 13 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Encapsulation Destination Address 6 Bytes Source Address 6 Bytes Type 2 Data 46-1500 Bytes CRC 4 Bytes Type 0800 2B IP Datagram 46-1500 Bytes Type 0806 2B ARP Request/Reply 28 Bytes PAD 18 Bytes

14. Ethernet and MTU As you can see there is a limit on the size of an Ethernet frame. This limits the number of bytes of data to 1500 bytes. This characteristics of the data link layer is called MTU, or Maximum Transmission Unite

15. Typical MTUs NetworkMTU (bytes) 16 Mbits /sec token ring (IBM) 17914 FDDI4352 Ethernet1500 Frame Relay1500

16. Ethernet and MTU When two hosts on the same network are communicating with each other, it is the MTU of the network that is important. But, when two hosts are communicating across multiple networks, each link can have a different MTU. The important numbers are the MTUs of the two networks to which the two hosts connect, but rather the smallest MTU of any data link that packets traverse between the two hosts.

17. Ethereal Exercise Do the following using Ethereal: Capture packets Filter IP packets. What is the value of the Ethernet type field and what does it mean? Clear the previous filter Filter ARP packets. What is the value of the Ethernet type field and what does it mean?

18. 18 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet and the OSI Model Not Included

19. IEEE 802.3 Frame