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Introduction to Networking: Difference between revisions
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This is a guide to basic networking, covering cables, hubs, switches, routers, IP, DNS, netmasks, firewalls and gateways.
It is meant as a basic guide and does not cover every aspect in great detail - just enough to get you started when setting up a network for the first time - especially if you have something like an ADSL router.<br>
The speed may change, but it is still effectively an ethernet network we are talking about.
The cables you will have heard of are
Cat 5 is basically a system of structure cabling, and a cat5 lead or patch lead is just part of that.
A patch lead is just a lead with a plug each end that connects things together.
Cat5 cabling does not generally have any cables over 100m long, and some types of network need shorter lengths to work at full speed.
The cable itself has 8 wires arranged as 4 pairs.
The fact that they are pairs is important to the way the electrical signals are carried.
Each pair has a colour: blue, orange, green and brown.
Each pair is two wires twisted together (see picture on right).
One is coloured (with maybe thin white stripes) and the other is white with thin coloured stripes.
The colours themselves don't matter - the electrical signals don't matter, but there are conventions which you should follow, and it is important that the pairs are not split.<br> <br>▼
▲The colours themselves don't matter - the electrical signals don't matter, but there are conventions which you should follow, and it is important that the pairs are not split.
[[Image:plug.gif|RJ45 plug]]If you look at the plugs on the end of the cable they are 8 way square, usually clear plastic and you can see the wires in the plug.
The colours normally used, are...
To make this work there are two types of sockets - the ones you find on a computer and the ones you find on a hub.
The two types of sockets are wired differently so that a straight cable can be used to connect between them correctly.
This means that at one end pins 1 and 2 are transmit and at the other end they are receive, so it works.
In some cases you need to connect a computer port to a computer port or a hub to a hub.
Some hubs have an alternative or switched port to allow hub to hub connections with a straight lead, but if this is not the case, or it is in use, you need a crossover lead.
1 to 3, 2 to 6, 3 to 1, and 6 to 2.
You can tell a crossover lead as the colours of the cables are different at each end, e.g.
one end normally starts White then Green rather than White then Orange as normal.
Cables give us a way to connect two devices together.
This is an uplink port.
The X means the wiring is crossed over so that the port works like a PC connection and a straight lead can be used to another hub.
It is important that if the port is an alternative rather than having a switch that you use one or the other socket and not both.
If you try and connect two uplink ports together, that would need a crossover lead as both ends are crossed over in that case.
You can use a crossover lead to connect two of the normal ports one two hubs together.<br>
This sort of networking works at various speeds.
The most basic is
Faster networks operate at
It is possible to get
rather than a hub.
Usually the switch/hub will automatically work out if the connection is
=== Switches ===
This means they cannot actually all talk at the same time.
WIth a hub, this is how it works - if one device is sending data, all of the others are receiving that data (and can't be sending data themselves).
This means on a
A switch on the other hand operates by receiving the message and sending it out on the right ports.
This means that if A is sending data to B, then C can be sending data to D at exactly the same time.
It means the speed is not shared between all of the devices.
The switch uses MAC addresses (see below) to work out which ports to send data to, and keeps track of what machines you have plugged in where.
As a switch can receive and send data independently on each port, it allows 10/100 switching.
I.e.
a mixture of
It can receive at
Obviously a switch has some memory internally to hold these messages.<br>
=== Full duplex ===
It is possible on
This means you can send and receive data at the same time on a port.
This again adds performance, but it only works where the PC can handle full duplex.
A managed switch simply means that the ports can be set up manually, and normally that data on the traffic being carried can be viewed (how busy each port is).
This means you could fix a port at
With hubs and switches, we now have a way of physically connecting '''several''' devices, such as a PC, physically together.
To achieve this the destination address is included in the start of the packet, and all of the devices seeing the packet look at that address to see if it is for them.
If it is not they ignore it, and if it is then they accept it and process it.
A switch uses this to work out from the address the port it should send the packet down instead of bothering all of the devices with every packet.
The address is a MAC address (Media Access Control).
Every network device as a unique MAC address fixed by the manufacturer of the network card.
MAC addresses are only used on the local network (LAN) to allow packets to be sent from one device to another.
A MAC address is actually a 48 bit number, and is usually written using hexadecimal, e.g.
00:03:79:12:33:57.
You don't normally have to deal with MAC addresses.
Now we have a way of connecting several devices, such as PC, physically together in a local network and a way that allows them to send specifically addresses packets of data to each other.
Unlike MAC addresses which are automatically allocated by the manufacturer of a network card, and which are used for addressing machines on a '''local'''
network, '''IP'''
addresses are allocated in a structure which allows packets to them to find their way from the other side of the world if necessary.
An IP address is a 32 bit number, and is normally written as a dotted quad, e.g.
192.168.0.1.
This means that the 32 bit number is broken in to 4 parts, each of 8 bits, and each part is written in decimal (a number from 0 to 255) with dots in between.
The address 192.168.0.1 is 11000000101010000000000000000001.
IP is a means to send packets to a specific IP address.
Again, like ethernet, the IP packet wraps up the data it contains, and adds addressing information and checking information to make an IP packet.<br>
This is because they are ''private addresses''.
They are just like any other IP addresses, except they will never be allocated to anyone, and so can safely be used in private networks.
The actual ranges are 192.168.x.x, 10.x.x.x, and 172.16-31.x.x.
If you are setting up a private network, and need some IP addresses, you should always use these ranges.
If you just make up addresses (e.g.
If you need to accept data from the internet (even if it is just the data in a web page you have requested) you need a public internet address.
With a modem this is typically allocated on the fly when you connect to the internet from your PC.
With a leased line or ADSL connection these may be permanently allocated to you.
The actual addresses are allocated by regional internet registries (RIRs), of which there are only 3 such registries.
There is one for the whole of Europe, called RIPE.
RIPE allocates large blocks of several thousand addresses at a time to local internet registries (LIRs) which are typically internet providers.
The whole world has to know where these addresses are to go to, so allocating large blocks to ISPs allows this to be done without too much difficulty.
The ISP will then assign addresses in small blocks to you.
The ISP makes sure that any packets that are sent for your addresses get to you, via your ADSL or leased line, or whatever.<br>
192.168.1.2 wants to send a packet to 192.168.1.3.
The Ip packet is created with these addresses, but to send on the local network this has to be put in to an ethernet packet.
The ethernet packet needs to say which MAC address to send the packet to.
In order to find the MAC address for an IP address on the local network, a special packet called an ARP request (Address Resolution Protocol) is sent.
This is a broadcast packet to all of the machines on the local network asking, e.g.
The machine with that address answers and tells it's MAC address.
This is remembered for a while, and any packets for that IP address can now be correctly addressed to the right MAC address.<br>
We have said that IP addresses have a structure.
Large blocks allocated to an ISP, and an ISP then routes a smaller block to a customer.
But how do you group IP addresses in to a
The answer is that a sequential set of IP addresses are treated as a block of addresses.
However, it is not as simple as saying that a certain customer has 1.2.3.5 to 1.2.3.17 as their addresses (that would be too simple <-:).
In practice a subnet is created.
A subnet is a group of addresses based on the binary IP address.
What happens is that part of the 32 bit address is assigned to a network.
e.g.
if we have machines all starting 192.168.5.
in a network (that's 192.168.5.0 to 192.168.5.255) then that is a 24 bit subnet.
This is because the first 24 bits, the 192.168.5 bit are the same for all machines on that network.
Looking at it in binary that network is 11000000101010000000010100000000 to 11000000101010000000010111111111, i.e.
the first 24 bits (110000001010100000000101) are the same, and the last 8 bits changed (from 00000000 to 11111111).
To define this a subnet mask is used.
This says which bits are the same, e.g. 11111111111111111111111100000000 which is 255.255.255.0.
All machines in a network have their own unique IP address with in the same block, and they also know their subnet mask.
This is important so that machines know if another address is on the same network.
If it is, then packets to another address can be sent directly and ARP used to find the MAC address to send them to.
If not on the same network, then the packet will have to be sent elsewhere (see below).
When the address block is 256 addresses, then it easy.
e.g.
When setting up a network, each machine has to have several parameters set.
We know the IP must be set uniquely for each machine, but all in the same group (subnet) of addresses.
The subnet mask must also be set on every machine, and we will see below that a gateway and DNS server are normally needed as well.
DHCP (Dynamic Host Configuration Protocol) is a way for one machine on a local network to take on the job of giving out addresses to other machines.
If you set up a DHCP server, you tell it a range of addresses to give out, and the details of subnet masks, gateway, DNS, and several other settings.
=== Getting off the network ===
So far we have seen how we can physically connect machines on a network; how they can send messages to each other by MAC address; how the can have IP addresses from a small group (subnet) and how (using ARP) they can send messages to each other by IP address.
By having a subnet mask, it is possible for a machine to work out that a packet is intended for a machine outside its own network.
e.g.
if the network is 192.168.5.0-255, a machine sending to 217.169.0.1 knows it is not on the same network - so what does it do.
Considering a PC with only its network connection, it has to send the IP packet in an ethernet packet, and so has to put a MAC address in that packet to say where it is going.
It knows the destination is not on the same network, so it knows there is no point sending an ARP to find the MAC address.
machine.
That is a machine that knows what to do with this packet.
This means that each machine needs a gateway address - the IP address of a machine on the local network which talks to the rest of the world.
Sending the packet to that machine (still addressed at the IP level to the original IP address), means that it can then be sent on its way via modem, ISDN, leased line, ADSL or whatever.,<br>
=== Routing ===
A routing table says which blocks of IP addresses go via what gateway.
You can set these on windows PCs (from DOS), but usually you will have one machine on your network that is a router, and it will have routing rules to say what goes down (say) the ADSL, what goes via another box (perhaps an ISDN router) and so on.
The PCs on the network can then live with just a gateway address and the gateway bounces the traffic to the right place.
In the internet there are lots of routes that say which blocks of IP addresses go where and this is how the IP addresses for you network get to come down the right line to your router.<br>
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So far we have seen how a network of networks (an internet) can be set up.
Each local area network has a small group of IP addresses (a subnet) and some gateways or routers carrying traffic off its network to the rest of the work.
The Internet is a collection of networks and links owned by ISPs and companies.
This is all very well if I want to send a packet to 217.169.0.1; It will get there.
What if I want to go to a web site? That has a name not a number.
What if I want to send an email - that uses a name not a number..
To solve this DNS (Domain Name Service) is used.
This allows names to be turned in to numbers.
Normally every PC will list one or two DNS servers (which have to be listed by IP address and not name or else you get a bit of a chicken and egg problem).
The only reason for two is resilience.
These are servers, usually provided by your ISP, which will look up names for you and get the addresses.
So, when you put www.me.uk in to a web browser, the browser uses DNS to look up that as 217.169.20.20 and then fetch the page.<br> <br>▼
▲So, when you put www.me.uk in to a web browser, the browser uses DNS to look up that as 217.169.20.20 and then fetch the page.
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With DNS we can call machines by names, and so we can go to web sites, send email, and communicate with the world<br>
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