IPv4 Troubleshooting in VPC

Key Point

• IPv4 cannot be disabled in AWS for VPCs or subnets.

• EC2 instance launches will still require an IPv4 address even if IPv6 is enabled.

• IPv6 availability does not prevent IPv4-related failures — most issues are caused by IPv4 exhaustion.

Common Issue

• Subnet runs out of available IPv4 addresses.

• AWS will block new instance launches until IPv4 capacity is restored.

Solution

• Expand the subnet by adding a secondary IPv4 CIDR block to the VPC.

• Reassign workloads or adjust subnet allocations to free addresses.

Example

• VPC CIDRs:
• IPv4: 192.168.0.0/24
• IPv4: 10.0.0.0/24
• IPv6: 2001:db8:1234:5678::/56

• Even if IPv6 has available addresses, the lack of free IPv4 addresses in 192.168.0.0/24 or 10.0.0.0/24 will cause instance creation to fail.

Recommendations

• Monitor IPv4 utilization regularly.

• Plan subnet sizing to match growth forecasts.

• Add secondary IPv4 CIDR blocks before reaching exhaustion.