Docker: Network configuration: How to customize the network bridge and use my own subnet / netmask / CiDR

In my last posting i described how to configure the network settings of a container via docker command line:

--net none
--net bridge

Now i want to try to change the subnet from the standard 172.17.0.0/16 to another ip range.

There are some tutorials out there which say:

docker run -it  --net bridge  --fixed-cidr "10.100.0.0/24"  alpine /bin/ash
unknown flag: --fixed-cidr

but this doesa not work any more.

First you have to create new network:

docker network create --driver=bridge --subnet=10.100.0.0/24  --gateway=10.100.0.1 mybrigde
6249c9a5f6c6f7e36e7e61009b9bde7ac338173d8e222e214a65b9793d36ad6c

Just do a verification:

docker network ls
NETWORK ID          NAME                DRIVER              SCOPE
a00386e6a5bc        bridge              bridge              local
9365e4a966d0        docker_gwbridge     bridge              local
9d9fa338a975        host                host                local
6249c9a5f6c6        mybrigde            bridge              local
9ff819cf7ddb        none                null                local

and here we go:

alpine:~# docker run -it  --network  mybrigde  alpine /bin/ash
/ # ifconfig
eth0      Link encap:Ethernet  HWaddr 02:42:0A:64:00:02  
          inet addr:10.100.0.2  Bcast:0.0.0.0  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:14 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:1156 (1.1 KiB)  TX bytes:0 (0.0 B)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

Removing the network bridge is easy:

docker network rm mybrigde

and narrowing the IP range can be done like this:

alpine:~# docker network create --driver=bridge --subnet=10.100.0.0/24  --ip-range=10.100.0.128/25 --gateway=10.100.0.1 mybrigde
b0ba1d963a6ca3097d083d4f5fd979e0fb0f91f81f1279132ae773c06f821396

Just do a check:

alpine:~# docker run -it  --network  mybrigde  alpine /bin/ash
/ # ifconfig
eth0      Link encap:Ethernet  HWaddr 02:42:0A:64:00:80  
          inet addr:10.100.0.128  Bcast:0.0.0.0  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:12 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:1016 (1016.0 B)  TX bytes:0 (0.0 B)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

The ip address of the container is set to 10.100.0.128 as configured with --ip-range 10.100.0.128/25.

If you are not familiar with the CIDR notation, just us this nice online tool (http://www.subnet-calculator.com/cidr.php):