Access List Evaluation Rules

The rules in access list are compared in a special sequence from top to bottom one by one. Where the rule matches to any of the condition the comparison is stopped and the process is carried out.

The phenomena of making an access list and then comparing rules is as similar to program a code of multiple “if else” statements. Just the data package that will come to router then it will be compared with the rules one by one and if it satisfies any of the rules then it will be permitted to pass through the network.

The beginner rule tells us that all traffic coming from network should be left unused. This rule does not affect the data packet in any way. Actually the router compares the next rule that whether it matches or not. When the data packet meets the specification of the rule which is “pass all the data” it will send all the data.

When the first rule is matched then other rules will not be compared. In this case the rule says to deny the incoming so the packet so it is dropped.

Note:

All the access lists end with a hidden command of “deny” that drops the data packet

If a packet reaches interface E0, than it will be allowed to enter because after the denial of the first rule, it will go for the second rule, if it is satisfied than it is permitted access to systems on network. When a packet comes with a source address and it does not satisfies the first rule and then goes for the second rule and then for the third but it fails every time to match. Than the hidden command of deny play its role and the packet of data is dropped.