|
|
As you will know by now if you have read these pages in any order, the
internet uses TCP/IP as its network protocol. The IP part of TCP/IP stands for
Internet Protocol. Each device attached to the internet at a given time must
have a unique IP number, that goes for your PC right now, the routers at BT,
the web servers and mail servers at your ISP - EVERYTHING.
|
What does an IP Number look like?
???.???.???.??? - is the short answer.
An IP number is made up of 4 blocks which can range from 0 to 255. Why 255
you ask? Well its based on binary numbers and a Byte (that's 8 bits or 2
nibbles!) Counting in base 2 or binary:
| Bit Value |
128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
Total
|
| Ex 1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
255 |
| Ex 2 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
| Ex 3 |
1 |
0 |
0 |
0 |
0 |
0 |
1 |
0 |
130 |
So, given that information you can see that there are around 4 billion
combinations here. However there are many exceptions, some of which are owned
by governments, large corporations and ISP's. Gconnect the business ISP have
what is called a Class C which is
213.130.137.0 up to 213.130.137.255 so we have 250 or so numbers to
ourselves. Note that nobody else can use these numbers in the outside world.
Internal and external IP numbers
|
|
With very few IP addresses left to allocate and thousands of new devices
being manufactured every day, how does the world keep up? The short answer is
that it doesn't, IP addresses are not limitless and the internet is going to run
into trouble eventually. This is not a point for us to concern ourselves with
as the better brains of the planet have been finding solutions for many years
now. In the interim we use IP numbers which are kept behind closed doors to
route traffic round local networks. The IP ranges are known as internal IP
numbers or private IP numbers. The ranges are listed below:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
|
|
The most common for a small network of less than 250 machines is the
192.168.0.0 range. For detailed information on setting up your network with
this range of addresses look on our "Setting up a network for ADSL" page.
In summary, these IP ranges are reserved and not allocated on the internet.
So users can use these ranges safely in the knowledge that nobody is going to
intercept incoming network traffic. However, these IP addresses cannot be
presented to the outside world by your network, for access to the web via
ADSL, Broadband, modem or leased line we must use a gateway which normally
takes the form of a router. Routers are discussed in another page.
If you are still with us so far, great. Network masks are a bit more
confusing, but in common use on nearly every network. It should be noted that
the mask is not absolutely necessary, but it does make the network run faster.
A subnet mask for a 192.168.1.1 type network will probably look like this:
222.255.255.0 or perhaps 255.255.0.0 depending on the number of devices on the
network.
|
Address Allocation for Private Internets
http://www.scit.wlv.ac.uk/rfc/rfc15xx/RFC1597.html
|
|
|
Go Back to the About ADSL Tech Page
|
|
What does a subnet mask actually do?
A subnet mask masks sections of an IP address to speed up address location
finding as such:
If the IP number is 192.168.0.1 and the subnet mask is 255.255.255.0 then
the first three segments of the IP number are masked. The table below shows 1
segment (our 192 value is the decimal number)
| Bit Value |
128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
Dec Value |
| Address Segment |
1 |
1 |
0 |
0 |
0 |
0 |
0 |
0 |
192 |
| Mask Segment |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
255 |
| Result |
|
|
|
|
|
|
|
|
|
So the 192 segment is totally masked, but what about with a different mask
number?
| Bit Value |
128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
Dec Value |
| Address Segment |
0 |
0 |
0 |
1 |
1 |
1 |
0 |
0 |
28 |
| Mask Segment |
1 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
128 |
| Result |
|
|
|
|
|
|
|
|
|
Any number over 128 is then blocked by the mask segment.
With a network mask in place covering the first 3 segments of an IP address
the lookup of a device on the network is limited to the last segment of the IP
address, which makes everything happen a lot faster.
In practical terms, if your network has less than 250 machines then use the
following range:
IP addresses 192.168.1.1 - 192.168.1.250
Subnet mask 255.255.255.0
How to find out IP Number information
You can find out the IP number assigned to your PC by typing IPCONFIG at the
command prompt, the result will look like:
|
|
Note this is on a Windows 2000 machine running a local network and an ISDN
adaptor. So what does the screen tell us?
The ethernet adaptor (ie the network card) has an IP number of 192.168.1.1
(which fits into the Internal IP range described earlier) and the subnet mask
it that of a small network, which is appropriate. The gateway is the method of
finding IP numbers not on the local network.
The PPP adaptor which is the ISDN
card has the gateway and IP number set to the same number, this is because the
modem acts as a gateway for the single machine. The subnet mask is set to
255.255.255.255 which prevents IP access to the rest of the network at the ISP
end of the connection. |
See if a device is available to the network
For this we use the PING command, for example: PING www.adsl-business.co.uk (by
name) or
PING 213.130.137.211 (by IP Number)
Either is perfectly okay, a DNS Server (discussed later) translates the name
into an IP number for us. Output looks like this:
|
|
