Which Rule Decides It? — TechKnowSurge

About this interactive

Every rule in a network ACL has the same parts: the action it takes, permit or deny, and the traffic it matches, a source, a destination and a port. The source or destination can be one host (all 32 bits must match, as in "host 10.10.10.10"), a network range (10.10.10.0/24 matches any address from 10.10.10.0 to 10.10.10.255), or "any", which matches everything. The port is the TCP or UDP port the packet is going to, or any. The router compares each packet with the rules in order. It starts at rule 1 and moves down, and the first rule whose source, destination and port all match decides: permit and the packet goes on, deny and it is dropped without a word. No later rule is read, so a broad permit written above a narrow deny decides every packet the deny was meant for. At the bottom of every list sits a rule you never typed: the implicit deny any. It is not shown, but it is always there, and it drops whatever matched nothing above it. A list that permits two services therefore denies everything else.

What you'll learn

Match a packet against the parts of each rule, its action, source, destination and port, reading a single host, a network range and "any" correctly
Process a network ACL as the router does: top to bottom, stopping at the first rule that matches, so a broad rule above a narrow one can decide a packet the narrow one was written for
Apply the implicit deny any rule: a packet that matches none of the written rules is dropped

Aligned to

CompTIA Security+

4.5 Given a scenario, modify enterprise capabilities to enhance security.

2.5 Explain the purpose of mitigation techniques used to secure the enterprise.

Key terms

Access Control Entry: An individual rule within an access control list that specifies whether to permit or deny access for a particular user, group, or traffic pattern matching defined criteria. A complete ACL is made up of an ordered sequence of ACEs.
ACL Rule Ordering: The sequential evaluation of ACL entries on a Cisco router, where the first matching rule is applied and subsequent rules are not evaluated; incorrect ordering can cause traffic to be permitted or denied unintentionally.
Implicit Deny: A foundational access control principle in which any traffic or request not explicitly permitted by a rule is automatically blocked. It appears as a hidden deny-all rule at the end of every access control list.
Access Control List: A set of rules that defines which users or systems are granted or denied access to a resource.
Explicit Rule: An access control rule that is directly stated and deliberately configured by an administrator.
Implicit Rule: An access control rule that is not directly stated but is automatically applied by default, such as a deny at the end of an ACL.

Topics

Access Control List Access Control Entry Acl Rule Ordering Implicit Deny Interactive Predict

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