Metcalfe’s first sketch of how Ethernet might work
Modern Ethernet
Over time the Ethernet specification has developed to take account of new developments.
The first big change was the dropping of the thick coaxial cables and the “taps”. In their place was either a network connection based on twisted pair cables or a thinner and cheaper coaxial cable. The original specification became known as 10Base5, the thinner coax system was known as 10Base2 and the twisted pair hardware as 10BaseT.
There are other standards that relate to the use of fibre optic connections but the important changes to the original standards are “speed”! The “10” in the names of the Ethernet standards relates to the transmission speed of 10Mbits/s, which was once thought fast enough for anything. However you have to keep in mind that the sort of information sent over local area networks has changed considerably in the past few years. In particular, where we once just shared files and perhaps sent a little email today we want audio and video links to be carried over the network.
You also have to remember that the 10Mbits/s is the capacity of the cable and this has to be shared, via the Ethernet CSMA/CD protocol, between all of the machines connected.
To meet the demands of a multimedia network the Ethernet specifications were improved to 100Mbits/s and 100Base-T or “Fast Ethernet” was born. This uses a twisted pair cabling and intelligent network hubs to allow older slower Ethernet devices to connect at their own speed.
However, even Fast Ethernet isn’t the last word. In 1998 a new Ethernet standard called Gigabit Ethernet was announced. Initially this was a fibre optic standard but a year later 1000Base-T was introduced and this works over four twisted pairs.
If you think 1Gigabit is fast enough, the standards committees have produced standards for 10Gigabit and faster. On another front, the growing use of wireless networks takes Ethernet back to where it came from, i.e. to ALOHAnet and ways of sharing the airwaves. Yes wireless LANs using WiFi make use of the same Ethernet protocol and it’s called 802.11.
That's about the current state of Ethernet but before we leave the subject we need to consider how it is mostly used today to build the Internet.
TCP/IP
The TCP/IP protocol, the way that the Internet works, is often thought of as the way that all networking works but the truth is rather more complicated. TCP/IP is a higher level protocol that can be used with a range of so called "transports" and Ethernet is just one of the many possible transport methods.
Small groups of machines are generally connected together using Ethernet hardware and protocols. Every Ethernet network adaptor has a unique numeric address that is allocated by the manufacturer when the card is made - its MAC address.
It is this address that is used to ensure that data packets from one machine get to the intended destination. The switches that you use to connect different machines together remember the MAC addresses of all of the machines connected to them. In this way they can work out how to get data from one machine to another.
The problem is that the Ethernet being based on MAC addresses isn't "routable". If a switch gets a data packet with a MAC address it doesn't recognize as belonging to a machine connected to it then i hasn't a clue what to do with it. MAC addresses are more or less random numbers and on their own they don't give any clue as to how to get to the machine with that MAC address.
The Internet being spread over such a large area and with so many machines has to use a routable addressing scheme. A routable address contains different sections that start with the general and get increasingly specific. A standard postal address is routable because it starts with the country, then city then street then house - if you read from the bottom up. An IP - Internet Protocol address is routable in this way and its structure allows a router to send it closer to its destination even if it doesn't know the exact destination.
So the Internet doesn’t work in terms of Ethernet MAC addresses but IP addresses that are derived from the URLs that you type.
Today the main role for Ethernet is to carry TCP/IP packets between machines on a local section of the network.
The way that this works is that the Internet data packet is “wrapped” by the Ethernet data packet which does the real work. The local switches use the MAC address to dispatch the packet to its destination in the usual way. If the IP address isn't a local machine but a router then the router "opens" the Ethernet packet and extracts the IP address which it uses to work out which router it should send the packet to.
So the strange thing is that in the local network packets are delivered by Ethernet and MAC address. When the packet needs to leave the local network it is the IP address that determines where it goes.
Essentially the Internet packet is just the data part of the Ethernet packet! In this sense perhaps it is fair to say that it is the Ethernet that actually drives the majority of the Internet!
The long distance links that make the Internet global don't always use Ethernet technology but it provides nearly all of the local connections that we use in one form or another.
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