Related concepts:

• Bridge: equivalent to a virtual switch, all devices connected to the bridge can communicate normally;

• veth pair: virtual network card pair (2 network cards), the sending and receiving data between the two network cards are consistent;

docker network:

Docker0 bridge: after installing and starting docker0, there will be a network card device of docker0 (this device is equivalent to a switch);

After the docker container is created, two virtual network cards will be created, one end is displayed in the host, the other end is eth0 in the container. These two network cards are virtual network card pairs.

Create two containers and execute ip a on the host. You can see that there will be two virtual network cards (starting with veth):

docker run -d --name nginx-1 nginx
docker run -d --name nginx-2 nginx

[root@ks-allinone ~]# ip a
...
3: docker0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default 
    link/ether 02:42:92:a1:07:20 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0
       valid_lft forever preferred_lft forever
    inet6 fe80::42:92ff:fea1:720/64 scope link 
       valid_lft forever preferred_lft forever
37: veth87e12aa@if36: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master docker0 state UP group default 
    link/ether 56:3f:71:00:b2:95 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet6 fe80::543f:71ff:fe00:b295/64 scope link 
       valid_lft forever preferred_lft forever
39: vethed473fd@if38: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master docker0 state UP group default 
    link/ether 66:f0:c4:e4:df:a9 brd ff:ff:ff:ff:ff:ff link-netnsid 1
    inet6 fe80::64f0:c4ff:fee4:dfa9/64 scope link 
       valid_lft forever preferred_lft forever

brctl show: You can see that both virtual network cards are bound to docker0 (equivalent to connecting to a switch via a virtual network card on the host).

[root@ks-allinone ~]# brctl show
bridge name	bridge id		STP enabled	interfaces
docker0		8000.024292a10720	no		veth87e12aa
							            vethed473fd

Note: After connecting to the same bridge (equivalent to connecting to the same switch), the container can communicate with each other.