Firewalls and packet filtering in Computer Networks - Step-by-Step Execution

Concept Flow - Firewalls and packet filtering

Incoming Network Packet

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Check Firewall Rules

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Packet Allowed

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Forward to Device

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End Process

A network packet arrives and the firewall checks it against rules. If it matches allowed rules, it passes through; otherwise, it is blocked.

Execution Sample

Computer Networks

Packet arrives
Firewall checks source IP
If IP allowed, forward packet
Else, block packet

This simple process shows how a firewall filters packets based on source IP rules.

Analysis Table

Step	Packet Source IP	Rule Checked	Rule Match?	Action Taken	Result
1	192.168.1.10	Allow 192.168.1.0/24	Yes	Forward packet	Packet sent to device
2	10.0.0.5	Allow 192.168.1.0/24	No	Block packet	Packet dropped
3	172.16.0.3	Allow 172.16.0.0/16	Yes	Forward packet	Packet sent to device
4	8.8.8.8	Allow 172.16.0.0/16	No	Block packet	Packet dropped
5	End	-	-	-	No more packets to check

💡 Packets are processed one by one; if no rule matches, packet is blocked by default.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Packet Source IP	None	192.168.1.10	10.0.0.5	172.16.0.3	8.8.8.8	No more packets
Rule Match	None	Yes	No	Yes	No	N/A
Action Taken	None	Forward	Block	Forward	Block	N/A

Key Insights - 3 Insights

Why does the packet from 10.0.0.5 get blocked even though it is a valid IP?

Because the firewall rules only allow packets from 192.168.1.0/24 subnet in step 2, and 10.0.0.5 does not match this rule, so it is blocked as shown in execution_table row 2.

What happens if a packet does not match any allow rule?

It is blocked by default. This is shown in steps 2 and 4 where packets not matching any allow rule are dropped.

How does the firewall decide to forward or block a packet?

It checks the packet's source IP against the rules. If a rule matches (Yes in Rule Match column), it forwards; otherwise, it blocks, as seen in the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what action is taken for the packet from 172.16.0.3 at step 3?

A Block packet

B Forward packet

C Ignore packet

D Log packet only

💡 Hint

Check the 'Action Taken' column at step 3 in the execution_table.

At which step does the firewall block a packet because it does not match the allowed subnet?

A Step 2

B Step 3

C Step 1

D Step 5

💡 Hint

Look for 'Block packet' in the 'Action Taken' column and check the corresponding step.

If the firewall rule allowed 10.0.0.0/8 instead of 192.168.1.0/24, what would happen to the packet from 10.0.0.5 at step 2?

A It would be logged only

B It would be blocked

C It would be forwarded

D It would cause an error

💡 Hint

Refer to the variable_tracker for 'Packet Source IP' and 'Rule Match' changes if the rule changes.

Concept Snapshot

Firewalls check incoming packets against rules.
Packets matching allow rules are forwarded.
Packets not matching are blocked.
Rules often filter by IP, port, or protocol.
Default action is usually to block unknown packets.

Full Transcript

A firewall acts like a gatekeeper for network traffic. When a packet arrives, the firewall checks it against a list of rules. If the packet's source IP matches an allowed range, the firewall forwards it to the device. If not, the firewall blocks the packet and stops it from passing through. This process repeats for each packet. If no rules match, the packet is blocked by default. This simple filtering helps protect networks from unwanted or harmful traffic.