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A Brief History of Ethernet
Bob Metcalfe invented Ethernet in 1973 while at the Xerox Palo Alto Research Center. (This same innovative place also fostered the invention of the laser printer and the graphical user interface, among other things.) Bob and his team developed and patented a "multipoint data connection system with collision detection" that later became known as Ethernet. Bob went on to form a company specifically dedicated to building equipment for this new protocol. This company eventually became 3Com, one of the largest network companies in the world. Luckily, Ethernet was released into the public domain so other companies could build to the specification. This was not true of Token Ring and most of the other network protocols of the day. If Ethernet had been kept proprietary or limited to only one company's hardware, it probably wouldn't have developed into the dominant standard it is today. It was eventually adopted as an official standard by the International Electrical and Electronic Engineers (IEEE), which all but assured it wide acceptance by corporate and government users worldwide. Other standards have been developed based on Ethernet, such as Fast Ethernet, Gigabit Ethernet, and Wi-Fi.
Ethernet handles both the physical media control and the software encoding for data going onto a network. Since Ethernet is a broadcast topology, where every computer can potentially "talk" at once, it has a mechanism to handle collisions—when data packets from two computers are transmitted at the same time. If a collision is detected, both sides retransmit the data after a random delay. This works pretty well most of the time. However, this is also a downside to the Ethernet architecture. All computers attached to an Ethernet network are broadcasting on the same physical wire, and an Ethernet card on the network sees all the traffic passing it. The Ethernet card is designed to process only packets addressed to it, but you can clearly see the security implication here.
Imagine if the way the postal system worked was that a bag containing all the mail was dropped off at the end of the street and each resident picked through it for their mail and then passed it along. (It might be interesting to see who subscribed to Playboy and who was getting the past due notices.) This fictional system is not very secure nor does it make efficient use of everyone's time, but that is essentially how Ethernet was designed.
Nowadays, most Ethernet networks are switched to improve efficiency. This means that instead of each Ethernet port seeing all the traffic, it sees only traffic intended for the machine plugged into it. This helps alleviate some of the privacy and congestion issues, but plenty of broadcast traffic still goes to every port. Broadcast traffic is sent out to every port on the network usually for discovery or informational purposes. This happens with protocols such as DHCP, where the machine sends out a broadcast looking for any DHCP servers on the network to get an address from. Machines running Microsoft Windows are also notorious for putting a lot of broadcast traffic on the LAN.
Other broadcast types are often seen on Ethernet LANs. One is Address Resolution Protocol (ARP); this is when a machine first tries to figure out which MAC address relates to which IP address (see the sidebar on MAC addresses in Chapter 3). Ethernet networks use an addressing scheme called Medium Access Control (MAC) addresses. They are 12-digit hexadecimal numbers, and are assigned to the card at the factory. Every manufacturer has its own range of numbers, so you can usually tell who made the card by looking at the MAC address. If a machine has an IP address but not the Ethernet address, it will send out ARP packets asking, "Who has this address?" When the machine receives a reply, it can then send the rest of the communication to the proper MAC address. It is this kind of traffic that make Ethernet LANs still susceptible to sniffer attacks even when they use switching instead of broadcasting all traffic to every port. Additionally, if hackers can get access to the switch (these devices are often poorly secured), they can sometimes turn their own ports into a "monitor" or "mirror" port that shows traffic from other ports.
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