H3C Technologies H3C SR8800 - Manual (Page 12)

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NOTE:
A wildcard mask, also called an inverse mask, is a 32-bit binary and represented in dotted decimal
notation. In contrast to a network mask, the 0 bits in a wildcard mask represent 'do care' bits, and the 1
bits represent 'don’t care' bits. If the 'do care' bits in an IP address are identical to the 'do care' bits in an
IP address criterion, the IP address matches the criterion. All 'don’t care' bits are ignored. The 0s and 1s
in a wildcard mask can be noncontiguous. For example, 0.255.0.255 is a valid wildcard mask.
ACL rule numbering
What is the ACL rule numbering step
If you do not assign an ID to the rule you are creating, the system automatically assigns it a rule ID. The
rule numbering step sets the increment by which the system automatically numbers rules. For example, the
default ACL rule numbering step is 5. If you do not assign IDs to rules you are creating, they are
numbered 0, 5, 10, 15, and so on. The wider the numbering step, the more rules you can insert between
two rules.
By introducing a gap between rules rather than contiguously numbering rules, you have the flexibility of
inserting rules in an ACL. This feature is important for a config order ACL, where ACL rules are matched
in ascending order of rule ID.
Automatic rule numbering and renumbering
The ID automatically assigned to an ACL rule takes the nearest higher multiple of the numbering step to
the current highest rule ID, starting with 0.
For example, if the numbering step is 5 (the default), and there are five ACL rules numbered 0, 5, 9, 10,
and 12, the newly defined rule will be numbered 15. If the ACL does not contain any rule, the first rule will
be numbered 0.
Whenever the step changes, the rules are renumbered, starting from 0. For example, if there are five rules
numbered 5, 10, 13, 15, and 20, changing the step from 5 to 2 causes the rules to be renumbered 0, 2,
4, 6 and 8.
Implementing time-based ACL rules
You can implement ACL rules based on the time of day by applying a time range to them. A time-based
ACL rule takes effect only in any time periods specified by the time range.
The following basic types of time range are available:
• Periodic time range—Recurs periodically on a day or days of the week.
• Absolute time range—Represents only a period of time and does not recur.
You may apply a time range to ACL rules before or after you create it. However, the rules using the time
range can take effect only after you define the time range.
IPv4 fragments filtering with ACLs
Traditional packet filtering matches only first fragments of IPv4 packets, and allows all subsequent
non-first fragments to pass through. Attackers can fabricate non-first fragments to attack networks.
To avoids the risks, the H3C ACL implementation:
• Filters all fragments by default, including non-first fragments.