networks: Docker containers capable of communicating in one subnet but not in the other


When I put my Docker containers in the 192.168.124.0/24 subnet, they can communicate. If I put them in 192.168.123.0/24, they can't. On both occasions, the applications are configured correctly for IP, literally, the only thing that is different is the subnet from which the addresses come.

Route departure

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         the.gateway 0.0.0.0         UG    0      0        0 enp9s0
10.0.0.0        *               255.255.255.0   U     0      0        0 enp10s0.500
10.215.60.0     *               255.255.254.0   U     0      0        0 enp9s0
link-local      *               255.255.0.0     U     1000   0        0 enp10s0.700
172.17.0.0      *               255.255.0.0     U     0      0        0 docker0
192.168.120.0   *               255.255.255.0   U     0      0        0 enp10s0.700
192.168.130.0   *               255.255.255.0   U     0      0        0 enp10s0.300
224.0.0.0       *               240.0.0.0       U     0      0        0 enp10s0.700

Iptables output -L

Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy DROP)
target     prot opt source               destination
DOCKER-USER  all  --  anywhere             anywhere
DOCKER-ISOLATION-STAGE-1  all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED
DOCKER     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Chain DOCKER (1 references)
target     prot opt source               destination
ACCEPT     tcp  --  anywhere             172.17.0.5           tcp dpt:8888
ACCEPT     tcp  --  anywhere             172.17.0.11          tcp dpt:2002
ACCEPT     tcp  --  anywhere             172.17.0.11          tcp dpt:2001
ACCEPT     tcp  --  anywhere             172.17.0.11          tcp dpt:cisco-sccp

Chain DOCKER-ISOLATION-STAGE-1 (1 references)
target     prot opt source               destination
DOCKER-ISOLATION-STAGE-2  all  --  anywhere             anywhere
RETURN     all  --  anywhere             anywhere

Chain DOCKER-ISOLATION-STAGE-2 (1 references)
target     prot opt source               destination
DROP       all  --  anywhere             anywhere
RETURN     all  --  anywhere             anywhere

Chain DOCKER-USER (1 references)
target     prot opt source               destination
RETURN     all  --  anywhere             anywhere

Of these I cannot see any problem. What could be the problem and how can I find out / fix it? The host is an Ubuntu 16.04