Merge pull request #17847 from TomSweeneyRedHat/dev/tsweeney/networkup

[CI:DOCS] Add network subnets info to network man page

docs/source/markdown/podman-network.1.md

@@ -35,5 +35,19 @@ so networks have to be created again after a backend change.
 | rm         | [podman-network-rm(1)](podman-network-rm.1.md)                 | Remove one or more networks                                     |
 | update     | [podman-network-update(1)](podman-network-update.1.md)         | Update an existing Podman network                               |
 
+## SUBNET NOTES
+Podman requires specific default IPs and, thus, network subnets.  The default values used by Podman can be modified in the **[containers.conf(5)](https://github.com/containers/common/blob/main/docs/containers.conf.5.md)** file.
+
+### Podman network
+The default bridge network (called `podman`) uses 10.88.0.0/16 as a subnet. When Podman runs as root, the `podman` network is used as default.  It is the same as adding the option `--network bridge` or `--network podman`. This subnet can be changed in **[containers.conf(5)](https://github.com/containers/common/blob/main/docs/containers.conf.5.md)** under the [network] section. Set the `default_subnet` to any subnet that is free in the environment. The name of the default network can also be changed from `podman` to another name using the default network key. Note that this should only be done when no containers are running.
+
+### Slirp4netns
+When Podman is run as rootless, the internet connectivity is provided with slirp4netns by default. Slirp4nents uses 10.0.2.0/24 for its default network. This can also be changed in **[containers.conf(5)](https://github.com/containers/common/blob/main/docs/containers.conf.5.md)** but under the `[engine]` section. Use the `network_cmd_options` key and add `["cidr=X.X.X.X/24"]` as a value. Note that slirp4netns needs a network prefix size between 1 and 25. This option accepts an array, so more options can be added in a comma-separated string as described on the **[podman-network-create(1)](podman-network-create.1.md)** man page. To change the CIDR for just one container, specify it on the cli using the `--network` opiton like this: `--network slirp4netns:cidr=192.168.1.0/24`.
+
+When using the default network as rootless, i.e. --network podman/bridge, then it also uses the same subnet as described above in addition to the slirp4netns subnet.
+
+### Podman network create
+When a new network is created with a `podman network create` command, and no subnet is given with the --subnet option, Podman will start picking a free subnet from 10.89.0.0/24 to 10.255.255.0/24. Use the `default_subnet_pools` option under the `[network]` section in **[containers.conf(5)](https://github.com/containers/common/blob/main/docs/containers.conf.5.md)** to change the range and/or size that is assigned by default.
+
 ## SEE ALSO
-**[podman(1)](podman.1.md)**, **[containers.conf(5)](https://github.com/containers/common/blob/main/docs/containers.conf.5.md)**
+**[podman(1)](podman.1.md)**, **[podman-network-create(1)](podman-network-create.1.md)**, **[containers.conf(5)](https://github.com/containers/common/blob/main/docs/containers.conf.5.md)**