Products
Webtrends Analytics 9.x
Webtrends Analytics 8.x
Issue
What is CIDR notation and how is it used by Webtrends?
Resolution
CIDR, aka CIDR notation, stands for “Classless Inter-Domain Routing” and works much like the current telephone numbering system in the United States. That is to say, in the United States, many people may share the same seven-digit phone number, and this is allowed due to the larger networks defined by area code.
Examples:
(215) 555-1212
(512) 555-1212
(503) 555-1212
(305) 555-1212
This area code hierarchy allows a larger regional network to be divided into many smaller networks, which then allows secondary backbone switches to be relied upon for the local regional traffic, and the primary switches would manage the general larger area traffic.
In prior methods Class A, B, and C addresses were given a standard prefix of 8, 16, or 24-bits. With the new method, CIDR, prefixes range from 13 to 32-bits. Because of this, IP blocks can be assigned in a range of 1 to 524,288 hosts.
Webtrends uses CIDR notation for IP addresses in two ways. The first use is IP address filtering. Often, an enterprise will use IP address filters to exclude traffic from certain users, groups, or machines, such as performance monitors. When excluding multiple ranges, the amount of data for an entry can become overwhelming. CIDR notation can be used to specify a range of specific IP addresses. The second way in which Webtrends uses CIDR Notation is with the configuration of Intranet Management. If certain blocks of IP addresses belong to certain departments the range (using CIDR notation, of course) may be assigned to the individual department.
Examples:
192.168.15.0/27 would assign 192.168.15.0 – 192.168.15.31 to Marketing
192.168.15.32/26 would assign 192.168.15.32 – 192.168.15.95 to IT
192.168.15.96/27 would assign 192.168.15.96 – 192.168.15.127 to Sales
From the example above, 0/27 represents the range from 0 – 31, with the number before the slash specifying the starting address (192.168.15.0) and the number after the slash specifying the number of bits total. Each octet is eight bits, so the value of 27 includes the first three octets (eight bits for each) and three bits of the last octet. These three bits assign the scope of the range from highest value down, so the bit values of 256, 128, and 64 are not used, leaving 32 (the next highest value) as the number of available addresses on the subnet: 192.168.15.0 – 192.168.15.31.
| CIDR Prefix | Dotted Decimal | # of Individual | # of Classful Length Addresses/Networks |
| /13 | 255.248.0.0 | 512K | 8 Bs or 2048 Cs |
| /14 | 255.252.0.0 | 256K | 4 Bs or 1024 Cs |
| /15 | 255.254.0.0 | 128K | 2 Bs or 512 Cs |
| /16 | 255.255.0.0 | 64K | 1 B or 256 Cs |
| /17 | 255.255.128.0 | 32K | 128 Cs |
| /18 | 255.255.192.0 | 16K | 64 Cs |
| /19 | 255.255.224.0 | 8K | 32 Cs |
| /20 | 255.255.240.0 | 4K | 16 Cs |
| /21 | 255.255.248.0 | 2K | 8 Cs |
| /22 | 255.255.252.0 | 1K | 4 Cs |
| /23 | 255.255.254.0 | 512 | 2 Cs |
| /24 | 255.255.255.0 | 256 | 1 C |
| /25 | 255.255.255.128 | 128 | 1/2 C |
| /26 | 255.255.255.192 | 64 | 1/4 C |
| /27 | 255.255.255.224 | 32 | 1/8 C |
| /28 | 255.255.255.240 | 16 | 1/16 C |
| /29 | 255.255.255.248 | 8 | 1/32 C |
| /30 | 255.255.255.252 | 4 | 1/64 C |
| /31 | 255.255.255.254 | 2 | 1/128 C |
| /32 | 255.255.255.255 | 1 | 1/256 C |