Example: Filtering a group of consecutive IP addresses
The steps below show what will be filtered when 192.168.2.81/30 is entered.
Step 1: Convert 192.168.2.81 to binary numbers. The binary numbers for the decimal numer 192.168.2.81 are:
11000000.10101000.00000010.01010001.
(For conversion, you can use the calculator of Windows. For Windows XP and Windows Vista, click <View> on the
calculator and click <Scientific>. For Windows 7 and Windows 8, click <View> on the calculator and click
<Programmer>. Then select first <Dec> and enter the decimal number, for example "2", and select <Bin> to
calculate the according binary number. You will receive "10". In the example above, 6 zeros were added before
the number 10 because every binary number must consist of 8 digits.)
The number “30” after the slash is referring to the first 30 digits of the binary numbers.
Step 2: Convert a few IP addresses before and after 192.168.2.81 to binary numbers. Then compare their first 30
digits to the binary numbers of 192.168.2.81.
a. Convert 192.168.2.80 to binary numbers. The binary numbers are 11000000.10101000.00000010.01010000. The
first 30 digits are the same with the binary numbers of 192.168.2.81, thus 192.168.2.80 will be filtered.
b. Convert 192.168.2.79 to binary numbers. The binary numbers are 11000000.10101000.00000010.01001111. The
first 30 digits are different with the binary numbers of 192.168.2.81, thus 192.168.2.79 will not be filtered. This also
means the IP addresses before 192.168.2.79 will not be filtered. Therefore, users can stop converting the IP
addresses before 192.168.2.79 to binary numbers.
c. Repeat the same procedure in “a” with the IP addresses after 192.168.2.81. Stop when the situation explained
in “b” happens. Namely, the 30th digit of the binary numbers of IP address 192.168.2.84 is different, and will not
be filtered.
As a result, the IP addresses 192.168.2.80 to 192.168.2.83 will be filtered when entering 192.168.2.81/30. But IP
addresses 192.168.79 and 192.168.84 will not be filtered when entering 192.168.2.81/30.
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Summary of Contents for GCI-F1576V
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