NAT Overload Causing Internet Access Failure

I once had a small office setup where users suddenly lost internet access, even though the router was up and running. Local traffic was fine, but anything beyond the LAN was a no-go. The culprit? A misconfigured NAT overload (PAT) setup.

🔍 Diagnosis

I started by checking the basics:

• show ip interface brief confirmed interfaces were up.
• ping 8.8.8.8 from the router worked, but not from internal hosts.
• show ip nat translations returned nothing—no active NAT entries.

That was the red flag. NAT wasn’t translating internal IPs to the public IP. I checked the config and found the NAT rule was missing the correct ACL and interface mapping.

✅ Solution

Here’s how I fixed it:

1. Defined the ACL for internal traffic:

access-list 1 permit 192.168.1.0 0.0.0.255

2. Configured NAT overload:

ip nat inside source list 1 interface GigabitEthernet0/0 overload

3. Set interface roles:

interface GigabitEthernet0/1 ip nat inside interface GigabitEthernet0/0 ip nat outside

4. Verified NAT translations:

show ip nat translations

Once this was in place, internal users could access the internet again. NAT entries started populating, and traffic flowed smoothly.

📚 Reference That Helped

The Cisco NAT Configuration Guide was my go-to. It clarified how ACLs and interface roles interact with NAT, and helped me avoid common pitfalls like forgetting to set ip nat inside/outside on interfaces.

This issue is deceptively simple but incredibly common—especially when routers are reset or configs are copied without adapting to the new network. It’s a great reminder that NAT isn’t just plug-and-play; it needs precise alignment between ACLs, interfaces, and overload rules.