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Chapter 3. Configuring 3Com VCX IP Telecommuting Module
Telecommuting Module
IP address
IP addresses are written as four groups of numbers with dots between them. The numbers must be between 0 and
255 (inclusive); for example, 192.168.129.17.
Mask/Bits
The binary system uses the numbers 0 and 1 to represent numbers. A binary digit is called a bit. Eight bits in the
binary system can represent numbers from 0 to 255.
The mask indicates how much of the IP address is used for the network address and the computers’ individual
addresses, respecitvely. A mask consists of 8+8+8+8 = 32 bits. Below is a mask with 26 bits set to 1, which means
that 26 bits of the IP address is locked to the network address and can’t be changed within the network.
Bits 11111111 11111111 11111111 11000000
No. 255 255 255 192
In the 3Com VCX IP Telecommuting Module, a mask is written either as the number of bits that are 1 or as four
numbers (0-255) with dots between the numbers.
Sometimes it can be convenient to give a group of computers a network name, such as Administration, or specify
that only a handful of computers can change the Telecommuting Module configuration.
You can form a group of computers with a network name, if the computers have consecutive IP addresses. In order
to do this, you must set the mask to indicate that the network group consists of those computers only. The lowest IP
address for these computers tells the network number of the group.
This is easiest to explain with a simple example. You have 7 computers that will make up a group called
Administration.
Take the nearest power of two above the number of computers you want to include: 2, 4, 8, 16, 32, 64, 128 or 256.
Since you have 7 computers, 8 is the nearest. In this example, one IP address is free for future use.
Give the computers consecutive IP addresses. Make the first IP address a multiple of the power of two number you
selected, but under 255. In the above example, this means 0, 8, 16, 24, 32, 40, 48 and so on, up to 248. You might
choose to start with 136 (17 x 8). This would give the computers the IP addresses 196.176.1.136, 196.176.1.137,
196.176.1.138, 196.176.1.139, 196.176.1.140, 196.176.1.141, 196.176.1.142 and 196.176.1.143.
One of the IP addresses is free and can be used for an eighth computer in the future. You must enter the first IP
address in the series, 196.176.1.136, in the Network/IP address field.
Now you must set the mask so that only the computers with these eight IP addresses are included in this network.
Take 256 and subtract the number of IP addresses in the named network. In the example, we would have 256-8 =
248. The complete mask is 255.255.255.248.
Now you have created a group of computers (IP addresses) that you can give a single name, such as Administration.
Table of netmasks.
No. of computers Mask Bits
1 255.255.255.255 32
2 255.255.255.254 31
4 255.255.255.252 30
8 255.255.255.248 29
16 255.255.255.240 28
32 255.255.255.224 27
64 255.255.255.192 26
128 255.255.255.128 25
256 255.255.255.0 24
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