12. GCP Networking Basics: Network Address Translation

In the interconnected world of computers and networks, IP addresses act like digital street addresses, ensuring information packets reach their correct destinations across the sprawling expanse of the internet. However, with the abundance of devices and the limited pool of IPv4 addresses, a dilemma arises – how can networks with private, internal addresses communicate with the outside world?

This is where Network Address Translation (NAT) steps in as a resourceful "translator." Consider it a gateway that mediates internet communication for a privately addressed network. While devices within the internal network utilize private IP addresses (those not directly routable on the public internet), NAT dynamically alters their source IP addresses when these devices initiate outbound connections. This enables communication with external internet-facing resources as if they were originating from a pool of public addresses.

At its core, NAT serves a primary purpose: facilitating outbound internet connectivity for devices within private networks, all while shielding their internal, non-routable IP addresses. With NAT, you can conserve those valuable public IP addresses, bolster security, and maintain control over how your private network interacts with the vast landscape of the public internet.

The Problem: Outbound Internet Without Exposure

Let's consider a common scenario in GCP deployments:

• Private Networking Power: Your virtual machines (VMs) reside within a secure VPC, shielding them from unnecessary direct access from the outside world. This is an important security best practice.

• The Need for Updates: These VMs must still reach out periodically: fetching packages, contacting an API, etc. Outbound communication can't be totally severed for a functioning application.

• IP Limitations: Not every internal VM needs or should have a public IP. Not ideal cost-wise, and more surface area for attackers to try if they DO get on your network somehow.

How Cloud NAT Solves This

Cloud NAT steps in elegantly, acting as your cloud network's "internet connection bouncer":

• Egress Only: Cloud NAT is about enabling OUTBOUND traffic without opening the INBOUND floodgates. Your internal resources get to contact the world but remain hidden from random probes and direct attacks.

• Shared Public IPs: VMs without individual public IPs "masquerade" behind a limited pool of IPs owned by the Cloud NAT service. This keeps IP usage costs lower, yet the outside world is none the wiser, replies come back!

• Flexibility: Choose whether NAT applies at subnet levels within your VPC or more granularly via tagging mechanisms as with firewall rules – this offers control over exactly who does or doesn't get NAT access.

Let's Get Analogous

Think of an office building. Internal staff may require internet access for daily tasks but shouldn't each have world-facing desks. Cloud NAT is like the singular 'door to outside': everyone uses it for egress, yet random passersby cannot wander in – the internal workings remain protected.

Practical Use Cases

• Budget-Conscious Deployment: Need a large cluster for batch processing that's spun up, runs on private-only VMs, then torn down? NAT saves lots on otherwise idle IPs when running

• Patching and Central Management: NAT can channel outbound traffic through security chokepoints (proxy with filtering lives upstream). Sometimes this compliance outweighs individual machine direct outbound

• Hybrid Networks: A few key "jump boxes" in a VPC have the public IPs, everything else relies on NAT. When extending on-prem into the cloud, this eases that transition with less sudden IP explosion needed.

Types of Cloud NAT in GCP

Here's the core distinction between these types, then we'll break down the nuances:

• Dynamic (Automatic): GCP manages it; ephemeral public IPs from a pool for your use as those private machines reach out. Hands-off, easiest.

• Static (Manual): YOU reserve/own specific public IPs and have those directly mapped to handle egress; useful when predictability is essential.

Dynamic NAT: Simplified Setup, Lower Predictability

• Ideal for: Workloads not needing consistent outbound IPs, low overhead management is appealing, less cost conscious about individual IP churn

• The Process: Your VM needing to reach out will (internally) be seen as any of a range of IPs NAT knows its "customer" of sorts. The outside server only sees an IP Google uses as the source

• Watch Out: This masquerading has consequences! Some services dislike rapidly changing source IPs as a security-minded human would... may lead to needing some support tickets filed with whatever is being contacted this way

Static NAT: Control With Greater Responsibility

• Ideal for: Specific services that dislike/are built to trust stable IP sources, regulatory need to know EXACTLY what is on the wire as public to prove to auditors... these lead to this choice

• The Process: Your VM with internal address 10.x.y.z is mapped to 35.a.b.c which YOU manage through Google still in reservation terms. Less mystery if logs get scrutinized for compliance

• Watch Out: Cost. Public IPs if not always in use are less optimal spend than the pool approach used behind the scenes with dynamic NAT. Plus, it's YOUR duty to make sure nothing with just 'internal' network gets to send on those without careful firewall combo use!