a hidden gem in docker 18.06 – define your base CIDR for networks

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Where you ever annoyed by the CIDR ranges used when a docker network was created without any further ipam spec (eg. in docker-compose.yaml)?

There is something hidden in the PR https://github.com/moby/moby/pull/36396: You are able to set the subnet CIDR from where docker networks are supposed to be created, plus you are able to define the size of the subnet,

Since I am a big fan of the 100.64.0.0/10 carrier grade NAT segment (it’s huuuge  and a cool alternative to 10.0.0.0/8) and it’s a private network.

So what needs to be done is running dockerd like

dockerd --default-address-pool base=100.96.0.0/11,size=26

or you’ll add something like this to your daemon.json file

{
"fixed-cidr": "100.64.0.0/23",
"default-address-pools":[
{"base": "100.96.0.0/11", "size": 26}
]
}

Notice the plural in the json file – it took me quite a while to add the plural 😉

Unfortunately, this cannot be found in the official dockerd documentation up until now. I just found it as a PR comment (see here).

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