CODEWITHSUNDEEP
dockerdocker-swarm
Stephen
Stephen

Reputation: 8810

container and node IP addresses in Docker Swarm

I am going through the Docker tutorials and I'm a bit confused why containers might have different IP addresses than the nodes containing them in a swarm. My confusion is based on the below diagram, from this page in the tutorial.

enter image description here

The bigger green boxes are the nodes in the swarm; they each have their own IP and load balancer, and externally they're listening at port 8080. I believe that the yellow boxes are containers/tasks in the my-web service. They are listening on port 80 and I guess the service is setup to map port 80 from each container to port 8080 externally.

That much I understand more or less, but I don't see why the container/task would have/need a different IP address from the node that it is running on. Can anybody explain this?

If I had to guess, it would be because each container is basically a VM and VMs need their own IP addresses and no two VMs can have the same IP address, therefore the container cannot have the same IP as the node. But I'm not sure if that explanation is correct.

Upvotes: 5

Views: 2444

Answers (1)

Gavin Miller
Gavin Miller

Reputation: 43825

I'm still fairly new to docker/containers myself, but it's my understanding that you're referring to internal IPs and external IPs. Namely that the 192.168.99.100-102 would be externally addressable (aka publicly available), whereas the 10.0.0.1-2 address are for internal addressing only.

The reason for the internal addressing is so that you can have a larger pool of ip addresses to work with for your containers, which is why the 10.0.0.0/8 address space is used. These nodes still need to be addressable, so that your load balancer can correctly distribute the load. And according to the wikipedia entry, you've got 16,777,216 available IPs which allows your swarm to scale to many many containers if you needed it. Whereas you only have a limited number of external IP addresses for your services to be hit on.

Upvotes: 3

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