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Vista Remote 2 Installation and Operations Manual
51
Address ranges
Although you can define ranges of addresses, due to the way the mask operates, there are
certain restrictions on the particular ranges that can be set. For any given address you can
encompass neighboring addresses in blocks of either 2, 4, 8, 16, 32, 64, 128, etc. and these
must fall on particular boundaries. For instance, if you wanted to define the local address
range:
192.168.142.67 to 192.168.142.93
The closest single block to cover the range would be the 32 addresses from:
192.168.142.64 to 192.168.142.95.
The mask needed to accomplish this would be: 255.255.255.224
When you look at the mask in binary, the picture becomes a little clearer. The above mask has
the form: 11111111.11111111.11111111.11100000
Ignoring the initial three octets, the final six zeroes of the mask would ensure that the 32
addresses from .64 (01000000) to .95 (01011111) would all be treated in the same manner.
See Net masks - the binary explanation for details.
When defining a mask, the important rule to remember is:
There must be no ‘ones’ to the right of a ‘zero’.
For instance, (ignoring the first three octets) you could not use a mask that had 11100110
because this would affect intermittent addresses within a range in an impractical manner. The
same rule applies across the octets. For example, if you have zeroes in the third octet, then all
of the fourth octet must be zeroes.
The permissible mask values (for all octets) are as follows:
Mask octet Binary Number of addresses encompassed
255 11111111 1 address
254 11111110 2 addresses
252 11111100 4 addresses
248 11111000 8 addresses
240 11110000 16 addresses
224 11100000 32 addresses
192 11000000 64 addresses
128 10000000 128 addresses
0 00000000 256 addresses
If the access control range that you need to define is not possible using one address and one
mask, then you could break it down into two or more entries. Each of these entries could then
use smaller ranges (of differing sizes) that, when combined with the other entries, cover the
range that you require.
For instance, to accurately encompass the range in the earlier example:
192.168.142.67 to 192.168.142.93
You would need to define the following six address / mask combinations in the IP access
control section:
Network/address entry Mask entry
192.168.142.67 255.255.255.255 defines 1 address (.67)
192.168.142.68 255.255.255.252 defines 4 addresses (.68 to .71)
192.168.142.72 255.255.255.248 defines 8 addresses (.72 to .79)
192.168.142.80 255.255.255.248 defines 8 addresses (.80 to .87)
192.168.142.88 255.255.255.252 defines 4 addresses (.88 to .92)
192.168.142.93 255.255.255.255 defines 1 address (.93)
Mask
Enter an IP network mask that indicates the range of addresses that will be allowed or denied
access. For instance, if only a single specified IP address were required, the mask entry would
be 255.255.255.255 in order to specify a single location.