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A simple guide to Networks

What is a network?

The term ‘network’ has two levels. We are familiar with the terms ‘a network of roads’ and ‘a business network’, but the networks we refer to here are different: physical and non-physical. You would have difficulty driving from London to Inverness just by talking about it, but it is by no means necessary (or indeed desirable…) for all of your colleagues to be physically connected to each other.

Network hardware

The nuts and bolts of any network are the computers and the cables. Lots of other bits are used and we will look at these individually later, but for now think of the two tin cans with the taut string between: a ‘network’ connection!

There are loads of options with networking and these can affect the hardware you install on your PC. The most common network is Ethernet connection. Ethernet is a type of carrier used by the PC to talk across networks, and this language can run on various cable types, each having particular characteristics. Don’t worry to much for now about the details of the cables, just remember that each type needs to connect to your PC and therefore needs a Network Interface Connection (Card). So, install the NIC into your computer, connect the cable, do the same at the other end and a network is born.

To actually connect two PC’s together the cable can connect directly (use a ‘Null’ cable) or to allow future growth a hub can be used. Plug your cable into the hub along with the other computer’s cable, and they will be able to connect and chat happily. Put several computers on the network through the hub and they can all chat away to each other.

So, hubs connect groups together, and some of these hub configurations can be quite complex. Lots of computers on the same hub can lead to lots of clutter with all those machines talking to each other, so larger companies use routers (also called bridges). Routers split groups of computers, and only allow traffic across the ‘bridge’ between the two groups if the message is for a computer in the other group. This means that one group can chat happily away without affecting the other group. This cuts down on network traffic but is only usual on larger networks.

Networking software

In order to have one PC talk to another there must be a physical connection to the network. This connection may eventually be by wire or through radio transmitters (wireless network), but you need to have your PC connected via its NIC to the network.

Hardware alone, of course, does nothing. Like the road from London to Inverness the aim is to move traffic from point A to point B. In order to control this traffic you have to have connection protocols and use very specific mechanisms to control the way that your data is sent and received. To do this multiple layers of software are used. There will be at least one layer of software to connect to your NIC card, then another layer of software to connect to the network and finally software to manage the traffic on the network.

Transmitting files

Let us say you want to send a spreadsheet to the accounts department over the network. It would create problems if the network just connected your two computers and kept everyone else off the network while they sent and received data. You also want to ensure that the file you send gets to the receiving PC without getting corrupted en route. To solve both problems the networking software chops your big file into thousands of little chunks called packets and the transport protocol keeps an eye on these packets.

Each machine on the network (and this will include network printers or other devices) has an address. This is the name by which your NIC is known and usually is in the format of four sets of three numbers e.g. 127.001.001.001. Sometimes a MAC address is used which is two letters or digits e.g. 78:C9:65:67. For ease of use we will use the computer names of Bert and Fred, not the numbers, for now. Data is transmitted between NIC cards and therefore between the computers.

What happens when you want to send a message between Fred and Bert?

First you have to logon or connect to your network. Once this is done a piece of software will pick up your file and chop it into packets. It then adds a bit of data to the front of the packet saying “I am for Bert from Fred and I am packet 1 of 1000”. At the end of each packet another bit of information is added saying “Bert, let me know if you got this bit from me, many thanks, Fred.” It then gets the next packet, repeats the entire header and tail but with the packet number 2 of 1000 then 3 and so on.

The software takes packet 1 and throws it onto the network. The packet goes around every NIC saying “Are you Bert”. When one responds “Yes” it says I have a message for you and delivers the packet, Bert sees the packet is from Fred and when it gets to the tail it replies to Fred saying “Got the message, send me your next packet” This is sent back via the network (and every PC on it) 'till it reaches Fred who understands the last packet got to Bert and then sends the next and next and so on till the entire file has been transferred and a message comes back saying “got all your packets, thanks for the file”.

The important point here is all these messages are flying backwards and forwards and going to every computer attached to the network hub. Any machine on the network with the right software can view every packet that is on the network at that time and this is where security issues come in to play.