Configuring Default Address Pools and Advertise Address in Docker Swarm

For example, if the default `10.0.0.0/8` range conflicts with already allocated address space in your network, then it is desirable to ensure that networks use a different range without requiring swarm users to specify each subnet with the `--subnet` command. To configure custom default address pools, you must define pools at swarm initialization using the `--default-addr-pool` command line option. This command line option uses CIDR notation for defining the subnet mask. To create the custom address pool for Swarm, you must define at least one default address pool, and an optional default address pool subnet mask. For example, for the `10.0.0.0/27`, use the value `27`. Docker allocates subnet addresses from the address ranges specified by the `--default-addr-pool` option. For example, a command line option `--default-addr-pool 10.10.0.0/16` indicates that Docker will allocate subnets from that `/16` address range. If `--default-addr-pool-mask-len` were unspecified or set explicitly to 24, this would result in 256 `/24` networks of the form `10.10.X.0/24`. The subnet range comes from the `--default-addr-pool`, (such as `10.10.0.0/16`). The size of 16 there represents the number of networks one can create within that `default-addr-pool` range. The `--default-addr-pool` option may occur multiple times with each option providing additional addresses for docker to use for overlay subnets. The format of the command is: ```console $ docker swarm init --default-addr-pool <IP range in CIDR> [--default-addr-pool <IP range in CIDR> --default-addr-pool-mask-length <CIDR value>] ``` The command to create a default IP address pool with a /16 (class B) for the `10.20.0.0` network looks like this: ```console $ docker swarm init --default-addr-pool 10.20.0.0/16 ``` The command to create a default IP address pool with a `/16` (class B) for the `10.20.0.0` and `10.30.0.0` networks, and to create a subnet mask of `/26` for each network looks like this: ```console $ docker swarm init --default-addr-pool 10.20.0.0/16 --default-addr-pool 10.30.0.0/16 --default-addr-pool-mask-length 26 ``` In this example, `docker network create -d overlay net1` will result in `10.20.0.0/26` as the allocated subnet for `net1`, and `docker network create -d overlay net2` will result in `10.20.0.64/26` as the allocated subnet for `net2`. This continues until all the subnets are exhausted. Refer to the following pages for more information: - [Swarm networking](./networking.md) for more information about the default address pool usage - `docker swarm init` [CLI reference](/reference/cli/docker/swarm/init.md) for more detail on the `--default-addr-pool` flag. ### Configure the advertise address Manager nodes use an advertise address to allow other nodes in the swarm access to the Swarmkit API and overlay networking. The other nodes on the swarm must be able to access the manager node on its advertise address. If you don't specify an advertise address, Docker checks if the system has a single IP address. If so, Docker uses the IP address with the listening port `2377` by default. If the system has multiple IP addresses, you must specify the correct `--advertise-addr` to enable inter-manager communication and overlay networking: ```console $ docker swarm init --advertise-addr <MANAGER-IP> ``` You must also specify the `--advertise-addr` if the address where other nodes reach the first manager node is not the same address the manager sees as its

This section details how to configure custom default address pools in Docker Swarm to avoid IP address conflicts, using the `--default-addr-pool` and `--default-addr-pool-mask-length` options. It provides examples of creating these pools with specific CIDR notations and subnet masks. Additionally, it explains the importance of configuring the advertise address for manager nodes, using the `--advertise-addr` option, to ensure proper communication and overlay networking within the swarm.