H3C Technologies H3C S10500 Series Switches - Manual (Page 11)

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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 comments and rule range remarks
You can add a comment about an ACL rule to make it easy to understand. The rule comment appears
below the rule statement.
You can also add a rule range remark to indicate the start or end of a range of rules created for the same
purpose. A rule range remark always appears above the specified ACL rule. If the specified rule has not
been created yet, the position of the comment in the ACL is as follows:
• If the match order is config, the remark is inserted into the ACL in descending order of rule ID.
• If the match order is auto, the remark is placed at the end of the ACL. After you create the rule, the
remark appears above the rule.
For more information about how to use rule range remarks, see the rule remark command in ACL and
QoS Command Reference for your device.
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
automatically 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 is numbered 15. If the ACL does not contain any rule, the first rule is
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.
Fragments filtering with ACLs
Traditional packet filtering matches only first fragments of packets, and allows all subsequent non-first
fragments to pass through. Attackers can fabricate non-first fragments to attack networks.