Hi@akhtar,
When you run Lambda outside your VPC it's likely running on a shared network out of your control.
When you run it in your VPC, the hosts running the function will attach a network interface provisioned inside your VPC, so you can route the traffic through a NAT gateway with fixed Elastic IPs.
Therefore the functionality of having fixed IPs is not directly implemented by Lambda but a side effect of running it in a VPC subnet routed through a NAT gateway.
Hope this will help.
Thank You