---
# System prepended metadata

title: LLM response for the question "Why is using an NAT gateway more cost effective than putting the resources in a public su
tags: [大語言模型回應, LLM responses]

---

# LLM response for the question "Why is using an NAT gateway more cost effective than putting the resources in a public subnet for small workload on AWS?"

https://hackmd.io/@brlin/SkqQ0uO4el

## Question

Why is using an NAT gateway more cost effective than putting the resources in a public subnet for small workload on AWS?

### Model #1

Gemini 2.5 Flash w/o thinking mode

#### Response

在 AWS (Amazon Web Services) 的情境下，使用私有子網路 (Private Subnets) 和 NAT 閘道 (NAT Gateways) 會被認為對於小型工作負載更「划算」，主要是因為它與另一種常見的替代方案進行比較：**為每個 EC2 執行個體分配公有 IP 地址 (Public IP Addresses) 並將其放置在公有子網路 (Public Subnets) 中。**

讓我們來詳細說明為什麼這種配置更划算：

##### 1\. NAT 閘道的計費方式：

NAT 閘道的主要計費項目是：

- **小時費用 (Hourly Charge):** 無論流量多寡，每小時都會收取一筆固定的費用（即使只有一個小流量，也需要付這個費用）。
    
- **處理的數據費用 (Data Processing Charge):** 每 GB 數據通過 NAT 閘道時會收取費用。
    

##### 2\. 公有 IP 地址的計費方式：

- **閒置 IP 地址費用 (Idle IP Address Charge):** AWS 對於「未與運行中的 EC2 執行個體關聯」的公有 IP 地址（或 EIP，彈性 IP 地址）收取費用。
    
- **數據傳輸費用 (Data Transfer Charge):** 從 EC2 執行個體傳輸到網際網路的數據，如果是通過公有 IP 地址，會計入數據傳輸費用。
    

##### 為什麼對於小型工作負載更划算？

現在，讓我們比較兩種情況：

**情境 A：使用私有子網路 + NAT 閘道**

- **優勢：**
    
    - **較低的公有 IP 地址費用：** 你只需要為 NAT 閘道（通常只有一個或幾個）分配公有 IP 地址。在私有子網路中的所有 EC2 執行個體都不需要自己的公有 IP 地址。這大大減少了因為「每個實例都有一個公有 IP」而產生的額外費用，尤其是在實例數量較多時。
        
    - **更簡化的網路管理和安全：** 將核心應用程式服務器放置在私有子網路中是標準的安全最佳實踐。它們無法直接從網際網路訪問，只能通過 NAT 閘道進行出站連接，並通常通過負載平衡器或跳板機（Bastion Host）進行入站連接。這減少了潛在的攻擊面。
        
    - **彈性 IP 地址效率：** 通常，NAT 閘道會使用彈性 IP (EIP)。雖然 EIP 在未關聯時會收費，但在這裡它是與 NAT 閘道關聯並使用的，所以這部分費用是合理的。
        
    
- **劣勢（在小型工作負載下相對不明顯）：**
    
    - 即使流量很小，NAT 閘道也會收取固定的小時費用。如果你的工作負載極度小，甚至只是偶爾需要出站連接，這個固定的小時費用可能看起來比較高。
        
    - 通過 NAT 閘道的數據傳輸費用。
        
    

**情境 B：為每個 EC2 執行個體分配公有 IP 地址（並放置在公有子網路中）**

- **優勢：**
    
    - 對於**極少數**且需要直接網際網路訪問的實例，可能看起來更簡單。
        
    
- **劣勢：**
    
    - **高昂的公有 IP 地址費用：** 如果你的小型工作負載涉及多個 EC2 執行個體，那麼每個執行個體都擁有一個公有 IP 地址的成本會迅速累積。即使這些 IP 地址是關聯到運行中的實例，它們仍然佔用了 AWS 的 IP 資源，並且可能涉及到更高的數據傳輸費用。
        
    - **安全風險：** 每個帶有公有 IP 的執行個體都直接暴露在網際網路中，增加了攻擊面。你需要為每個執行個體配置安全組和網路 ACL，管理起來更複雜，風險更高。
        
    - **數據傳輸費用：** 雖然兩種方案都有數據傳輸費用，但在公有子網路中直接處理出站流量，有時在某些計費模型下會導致整體成本更高。
        
    

##### 總結：

對於**小型工作負載**，儘管 NAT 閘道有固定的小時費用，但它提供了一種集中的、安全且成本效益高的方式來處理所有私有執行個體的出站網路流量。它避免了為**每個**私有執行個體分配一個公有 IP 地址所帶來的累積成本和安全複雜性。

因此，比較來看，與為每個需要網際網路訪問的 EC2 執行個體分配公有 IP 相比，**集中使用一個或少數幾個 NAT 閘道來處理多個私有子網路中執行個體的出站流量，從整體成本和安全性角度來看，對於小型到中等規模的工作負載是更「划算」的選擇。** 這裡強調的「划算」是針對 IP 地址管理、安全性和長期營運成本而言的。

### Model #2

Gemini 2.5 Flash w/ thinking mode

#### Response

You're asking a great question that gets at the core of AWS networking cost optimization for smaller setups. While it might seem counterintuitive at first (you're paying for another service), a NAT Gateway often becomes more cost-effective for small workloads with **multiple instances needing outbound internet access** when compared to putting all those instances directly in a public subnet.

Here's a breakdown of why:

- **Elastic IP (EIP) Cost Savings:**
    
    - **Public Subnet Approach:** If you place EC2 instances directly in a public subnet and they need public IP addresses to initiate outbound connections, each instance will need its own public IP. While a public IP automatically assigned to an instance is "free" as long as the instance is running and associated with it, if you use an **Elastic IP (EIP)**, it's free only if it's associated with a running instance and actively being used. If you allocate an EIP but don't associate it, or associate it with a stopped instance, or it remains idle, **AWS charges you for that EIP**. For small workloads, it's easy to forget about idle EIPs, leading to unexpected costs.
        
    - **NAT Gateway Approach:** With a NAT Gateway, **all your private instances share a single Elastic IP address** for their outbound internet traffic. This eliminates the need for each individual instance to have its own EIP, drastically reducing potential EIP charges, especially if you have several instances.
        
    
- **Managed Service Benefits (vs. Self-Managed NAT Instance):**
    
    - A common alternative to a NAT Gateway is to run an EC2 instance yourself and configure it to act as a NAT device (a "NAT Instance").
        
    - **Self-Managed NAT Instance:** This incurs the full cost of the EC2 instance (compute, storage, and a separate EIP), plus the operational overhead of managing it (OS patching, monitoring, ensuring high availability, scaling). For a small workload, a single NAT instance can become a bottleneck or single point of failure.
        
    - **NAT Gateway:** AWS handles all the underlying infrastructure, scaling, and high availability. You simply pay an hourly charge for the NAT Gateway and a data processing charge for traffic flowing through it. This eliminates the need for you to provision and manage an EC2 instance for this purpose, leading to significant savings in management overhead and often raw compute cost for a reliable setup.
        
    
- **Enhanced Security Posture:**
    
    - **Public Subnet Approach:** Instances in a public subnet are directly exposed to the internet. While you can secure them with Security Groups and Network ACLs, they are still directly addressable from the public internet, which increases their attack surface.
        
    - **NAT Gateway Approach:** Instances are placed in **private subnets** (which cannot be directly accessed from the internet). All outbound internet traffic from these instances is routed through the NAT Gateway. This significantly reduces the attack surface of your instances, making them inherently more secure. While not a direct cost saving, a better security posture reduces the risk of security incidents, which are extremely costly to remediate.
        
    
- **Simplified Networking & Scalability:**
    
    - It's simpler to configure private subnets and route outbound traffic through a central NAT Gateway.
        
    - NAT Gateways are highly available and scale automatically to handle burst traffic, meaning you don't have to over-provision or worry about performance bottlenecks for outbound traffic, even for small workloads that might have occasional spikes.
        
    

**When a NAT Gateway might be less cost-effective for a small workload:**

- **Single Instance, Low Traffic, and No EIPs:** If you have only one EC2 instance that needs occasional outbound internet access, and you're comfortable with it having a public IP (not an EIP you pay for), the hourly cost of the NAT Gateway and data processing might slightly exceed the "free" public IP scenario.
    
- **Extremely High Outbound Traffic:** For very large volumes of outbound data transfer, the "Data Processing" charge (per GB processed by the NAT Gateway) can add up. In such extreme cases, direct egress from public instances might sometimes be cheaper, but this is rare for "small workloads."
    

**In summary, for small workloads with multiple instances that need outbound internet access but not inbound internet access (which is the typical use case for backend application servers, databases, etc.):**

- The **Elastic IP consolidation** for all your private instances is a major cost saver.
    
- The **managed service** aspect avoids the compute and management overhead of running your own NAT instance.
    
- The **improved security** is a significant, albeit indirect, cost benefit.
    

This makes the NAT Gateway a highly attractive and often more cost-effective solution for creating secure, scalable, and manageable private subnets for your compute resources on AWS.
