I am following Chapter 4: Multiple computers ROS networks from "Multi-master ROS systems" written by Sergi Hernandez Juan and Fernando Herrero Cotarelo.

My first setup looks like this:
computer1a is connected via wifi to the router which act as the common network (network1)
computer1a is connected to computer1b via ethernet cable
computer1b has its own network (network2)

The idea is that computer1a will act as the gateway to the common network (network1) and roscore will be run from computer1b. There will be another set of system (computer2a and computer2b) for the final setup. But for testing purpose I am only testing computer1a and computer_1b.

IP of computer1a (network1): 192.168.0.50
IP of computer1a (network2): 192.168.155.30
IP of computer1b (network2): 192.168.155.10

I have made sure that multicast is enabled in both computers.

When I ping 224.0.0.1 from computer1a, I only see respond from 192.168.0.50 (its own IP). Meanwhile when I ping 224.0.0.1 from computer1b, I only see respond from 192.168.155.10 (its own IP).

Afterwards I changed the setup so that my second setup looks like this:
computer1a is connected via wifi to the router which act as the common network (network1)
computer1b is connected via ethernet to the router which act as the common network (network1)

IP of computer1a (network1): 192.168.0.50
IP of computer1b (network1): 192.168.0.51

When I ping 224.0.0.1 from computer1a, I get respond from both 192.168.0.50 (computer1a) and 192.168.0.51 (computer1b). But when I ping 224.0.0.1 from computer1b, I still only get respond from 192.168.0.51 (its own IP).

I dont have much knowledge in computer network and would appreciate any help or guidance I can get in troubleshooting this.