AWS costs are notoriously difficult to compehend. The networking costs even more so.

It personally took me a long time to research and wrap my head around it - the public documentation isn't clear at all, support doesn't answer questions instead routes you directly to the vague documentation and this subreddit has a lot of old threads that contradict each other, without any consensus - so the only reliable solution is to test it yourself.

So I did.

Let me share all I learned so you don't have to go through the same thing yourself.

Data Transfer

For simplicity, we will be focusing only on EC2 transfers. Any data that goes out of your EC2 or into your EC2 instance is liable to get charged.

Whether it does, depends a lot on the destination / source of the data.

Transfer Outside AWS (so-called Internet Transfer)

This is called an internet charge. It captures data transfers between AWS and the internet.

The internet can mean:

• ☁️ other clouds (GCP, Azure)

• 🤖 on-premise environments

• 🏠 your home town’s ISP

• 📱 your phone’s cellular data

• etc.

Internet Ingress

✨ in few words: data coming from the internet into your AWS EC2 instance.

💸 charged: nothing

Ingress is infamously free across all major cloud providers. They’re incentivized to do that because it locks you in.

Internet Egress

✨ in few words: data going out of your EC2 into the internet.

💸 charged: $0.05/GB-$0.09/GB in EU/USA. Larger charges in other regions.

This can end up expensive. If you’re egressing just 1 MB/s consistently, it’ll cost you $2731 a year.

(Note there’s also Direct Connect that can end up offering cheaper internet traffic prices for certain on premise environments.)

Transfer Within AWS

Cross-Region Costs

✨ in few words: data flowing between two EC2 instances in different regions.

💸 charged: varying rates on egress (the instance sending data). ingress is free.

The cost here is very specific on the region-to-region pair.

This can be:

• as close as Oregon → Northern California

• as far as Oregon → Cape Town

Prices vary significantly. It isn’t strictly correlated with geographical distance.

For example:

• 1 TB sent from us-west-2-sea-1 (Seattle):

  • → ~700 miles (1140 km) → us-west-1 (N. California) costs $20.48 ($0.02/GB)

  • → ~2357 miles (3793 km) → us-east-1 (N. Virginia) costs $0

    • but sending 1 TiB back from us-east-1 costs $20.48 ($0.02/GB)

• 1 TB sent from us-west-2 (Oregon):

  • → ~10,244 miles (16,487 km) → af-south-1 (Cape Town) costs $20.48 ($0.02/GB)

    • but sending 1 TiB back from af-south-1 costs $150 (7.3x more @ $0.147/GB)

Same-Region Costs

Within a region, we have different availability zones. The price depends on whether the data crosses those boundaries.

Cross-AZ

Costs a total of $0.02/GB. In all cases. There is no going around this charge.

✨ in few words: data flowing between two EC2 instances in different availability zones.

💸 charged: $0.01/GB on ingress (instance receiving data) & $0.01/GB on egress (instance sending data)

If the data transfer is done cross-account then the bill is split between both AWS accounts.

Same-AZ

This is where a lot of confusion can come.

✨ in few words: data flowing between two EC2 instances in the same availability zone.

💸 charged: depends on IP type.

👉 ipv4: free when using private IPs.

👉 ipv6: free when inside the same VPC, or is VPC-peered.

Everything else is $0.02/GB. In other words - using public ipv4 addresses always results in a cross-zone charge, even if the instances are in the same zone. Crossing VPC boundaries using IPv6 will also result in a cross-zone charge, even if the instances are in the same zone.

Private IPs & Cross VPCs

A VPC is a logical network boundary - it doesn’t allow outsiders to connect to it. VPCs can be within the same account, or across different accounts (e.g like using a hosted MongoDB/ElasticSearch/Redis provider).

Crossing VPCs therefore entails using the public IP of the instance. That is, unless you create some connection between the networks.

This affects your same-AZ charge - but the documentation on this is scarce.

• AWS only ever confirms that same-AZ traffic through the private IP is free, but never mentions the cost of using public IP.

• There is a price distinction between IPv4 and IPv6, and it reads unclearly.

Even on this subreddit, I read some very wrong thoughts on this. It was really hard to find a definitive answer online. In fact, I didn’t find any. There were just a few threads/souces I could find over the last few years, and all had conflicting answers:

• 28 upvote replies implied you’ll pay internet egress cost if you use the public IP

• more replies assuming internet egress charges if using public IP

• even AWS engineers got the cost aspect wrong, saying it’s an intenet charge.

I ran tests to confirm.

So you can take this post as the definitive answer to this question online. I also posted and created some graphics around this in my newsletter - since I can't share images on Reddit, if interested - check the post out.