Thinking Like a Network Admin

September 16, 2025 - Reading time: 3 minutes

When you’re given a narrative like the BrightSide Graphics scenario, the goal isn’t to immediately start typing commands. Instead, you work backward from the constraints and requirements in the story to figure out the right tools and configuration.

In real life, you’ll often have to read between the lines — the boss or a client rarely says “configure PAT with overload on the interface and an inbound ACL.” They give you symptoms, policies, and limitations. Your job is to translate those into the exact features and commands that meet all needs without breaking anything else.

For this scenario, your reasoning process goes something like this:

Step-by-Step Thought Process

Identify the core requirement from the setup.
Many inside hosts, one public IP → this is clearly a NAT situation, specifically PAT for one-to-many translation.

Spot any constraints that affect the NAT type.
The ISP address is dynamic, not static. That means hardcoding the public IP into NAT rules is risky — instead, bind NAT to the outside interface so it adjusts automatically.

Define the NAT zones.
The LAN interface is the “inside” (Fa0/1), the ISP interface is the “outside” (Fa0/0). Without these roles, NAT won’t function.

Decide how to match traffic that needs translation.
Every LAN host in 192.168.1.0/24 should be eligible. The simplest match is a standard ACL with that subnet.

Apply security policy clues.
“Only traffic we ask for should come in” → block unsolicited inbound traffic. On Cisco IOS, that means applying an inbound ACL to the outside interface that allows only established TCP sessions and ICMP replies.

Sequence the tasks logically.
Configure interfaces & roles → create ACL to match inside subnet → apply PAT overload on outside interface → add inbound ACL for return traffic only → verify.

Expected Configuration (with explanations)

Step 1 — Interfaces & NAT Roles

interface FastEthernet0/1 
 ip address 192.168.1.1 255.255.255.0 
 ip nat inside 
 no shutdown
! Fa0/1 is the LAN; mark it as inside for NAT.

interface FastEthernet0/0
 ip address dhcp
 ip nat outside
 no shutdown
! Fa0/0 faces the ISP; it gets a dynamic address and is the outside for NAT.

Step 2 — Match the LAN for Translation

access-list 1 permit 192.168.1.0 0.0.0.255
! Selects all inside hosts that should be translated.

Step 3 — PAT that Follows the Interface

ip nat inside source list 1 interface FastEthernet0/0 overload
! Overload on the outside interface so NAT always uses whatever public IP 
! the ISP assigns to Fa0/0.

Step 4 — Allow Only Return Traffic from the Internet

access-list 102 permit tcp any any established 
access-list 102 permit icmp any any echo-reply

interface FastEthernet0/0 
 ip access-group 102 in
! established = inbound TCP replies only; echo-reply = allow ping responses
! everything else unsolicited is denied.

Final Configuration (Expected Answer)

! Step 1: Interfaces and NAT roles
interface FastEthernet0/1
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
 no shutdown

interface FastEthernet0/0
 ip address dhcp
 ip nat outside
 no shutdown

! Step 2: Match inside subnet
access-list 1 permit 192.168.1.0 0.0.0.255

! Step 3: PAT using outside interface
ip nat inside source list 1 interface FastEthernet0/0 overload

! Step 4: Inbound ACL for return traffic only
access-list 102 permit tcp any any established
access-list 102 permit icmp any any echo-reply
interface FastEthernet0/0
 ip access-group 102 in

Verification

show ip nat translations
show ip nat statistics
show access-lists 102

You should see dynamic NAT entries using the Fa0/0 address, and ACL hit counters increment on the permitted lines as users browse.