Home Network

Basic Concepts


These days, old computers are easy to come by : as people and companies continue to buy new PC's to match 'today's requirements' in HD space, RAM and CPU speed, there's agrowing second hand market where you can get 5 year old computers for next to nothing, and they'll still be sufficient for every day use. Even older ones, like an early pentium or a 486 can often be put to good use.

This allows you to collect a couple of usefull machines, make a home netwok, experiment with networking, setting up servers and clients, even trying to 'hack' one of your own systems - no need to mess with some innocent guy's PC on the internet.
You may also be tempted to try and get all these computers on the internet, preferably simultanously. All that is possible if you know a few things about networking and tcp/ip.

Hardware

At the risk of being obvious (this is probably common knowledge for someone who's trying to get network connected to the internet) : all these computers will need to have network interface cards connected to each other with cables.

If you're using only 2 computers, you can connect them with a cross-linked network cable i.e. a network cable that connects the 'sending' from one computer to the "receiving" from the other. For 3 or more computers, you'll need a hub, a small box where the 3 or more cables come together and connect to each other. The hub then also makes sure that the sending and receiving ends are correctly connected to each other, so you'll have to use 'straight' network cable.

If you want internet access, 1 computer will need a device to make an internet connection : a modem, an ISDN modem, a network adapter to connect it to a cable modem, ...
This computer will have to act as router (or proxy server or another method pf internet connection sharing). Therefore, it will need

  1. one network adapter, connected to the home network (LAN)
  2. one network adpater, connected to the internet (through) the cable modem.
    This can be replaced by a a modem of some kind, depending on what kind of internet connection you have. This is the WAN side.

Crossover vs. Straight Through Cables - (When to use what)

Note: NIC = Network Interface Card, HS = Hub/Switch

Use a crossover cable in the following situations:

Use a straight through cable (normal) in the following situations:

Some nicer hubs and switches allow you to choose (via a switch above the port, or possibly through software control) if the uplink port acts like a normal port (like any other on the back of the device) or like an uplink port (RTS/RTR crossed and Send/Recieve crossed).

Crossover cables are not any different from normal cables except that one end has some wires switched so that two devices aren't sending on the same line and listening (to nothing) on the same line.

source : Freesco Support Forums :: FAQ and MemeM :: SmartFAQ.

If you need to sort out existing cable and wiring or want to add something like additional modems, telephone outlets etc, this Cables and Connectors Survival Guide may come in handy.

Brief intro to TCP/IP addresses

The client(s) and your router need to be talking to each other using TCP/IP. In simple terms : You need to have TCP/IP, and all PC's need to have a unique IP address and be in the same "subnet". The side of the router that is connected to your LAN (local area network, i.e. your network of PC's that you want to connect to the internet), needs to be part of this network as well.

The WAN side of the router needs to be configured to match your internet provider's configuration re. IP addresses, subnet mask, (and proxy, DNS, DHCP etc if appliccaable - please refer to your provider's documentation).

An IP address looks like this : 142.36.244.1, i.e. 4 numbers from 0 to 255
A subnet mask may look like this : 255.255.255.0.

An IP address consists of a network portion and a host portion. The network portion refers to the network that the computer belongs to. It is also used for 'routing decisions' : a message to a computer on an other network -i.e. a message that needs to be sent to a computer that has a network portion in it's IP address that is different from the sender's network - will be routed out of the local network, while messages where the network portion of both sender's and receiver's IP address are identical, will remain inside the local network.

To know what is the network part of the IP address, you need to look at the subnet mask. Let's say you have 192.168.0.1 as address, 255.255.255.0 as subnet mask.
The 3 groups of "255" in the subnet mask indicate that the 3 first groups of the IP address represent the network, and the 1 remaining group of numbers therefore is the 'host portion' of the address, and represents the computer.
If you followed this, you'll now understand that 192.168.0.1
192.168.0.2
192.168.0.3
192.168.0.4
(and so on to 192.168.0.254 ; you can not use 0 or 255 as 'host portion' of an IP address)
are all valid IP addresses for this network. You just pick a different one for every PC you want add to the network. With this subnet mask you can thus connect 254 computers to each other and to the internet. Note that in order for this to work, all computers need the same subnet mask.

Likewise, If you'd have 172.16.0.0 as address and 255.255.0.0 as subnet mask, only the 2 first groups (172.16.) represent the network, the two remaining groups represent the 'host', and as each can be a number from 1 to 254, you can make 254x254 = approx. 65.000 valid addresses, all starting with 172.16. They will all have 255.255.0.0 as subnet mask.

BUT

There's also a few buts.
We all know that computers use binary numbers, and that these binary numbers represent 'bits'. Same thing for IP addresses abd subnet masks. Thise has some peculiar side effects if you go deep in to IP addressing and subnetting. An interesting expose on the details of IP addresses, subnetting and connecting a LAN to the internet can be found in Daryl's tcp/ip primer (there's a link in the Network section of the library).

The're some kind of a rule for IP addresses on private networks, such as your home network : you should only use addresses fom the following ranges :

These addresses are reserved for use on private networks, local area networks. You'll understand that there could be a machine with 10.0.0.1 on your home network, an other computer with 10.0.0.1 on your neighbour's home network, another machine with 10.0.0.1 in the town hall's office network, and so on. Therefore, these addresses are not unique across the internet. That's why you need a 'gateway' that has a network card (or a dial-up adapter, or ...) with a public IP address. It also means that a router on the internet will be unable to exactly locate a machine with address 10.0.0.1 : there are so many .... For that reason, these private addresses are sometimes called 'unroutable', and as a nice side effect, this protects computers with private addresses from external attacks : the bad guys on the internet can not connect to a PC with an unroutable address, an address that can not be located.

Internnetworking : Connecting a local network to the internet