Why do I need to specify the default gateway for two network namespaces in the same subnet?

Question

I am learning about network namespaces. I created two network namespaces - red and blue - on my system and attached a veth pair to connect between them - veth-red and veth-blue. I assign IP addresses to each of those interfaces - 192.168.1.1 and 192.168.1.2. I then bring the interfaces up. However they can't ping each other unless I specify a Default Gateway. I though you only need to specify a default gateway if they are on different networks. The code is here:

# Create network namespaces
ip netns add red
ip netns add blue

# Create veth pair
ip link add veth-red type veth peer name veth-blue

# Add veth to respective namespaces
ip link set veth-red netns red
ip link set veth-blue netns blue

# Set IP Addresses
ip -n red addr add 192.168.1.1 dev veth-red
ip -n blue addr add 192.168.1.2 dev veth-blue


# Bring up interfaces
ip -n red link set veth-red up
ip -n blue link set veth-blue up

# Bring up Loopback interfaces 
ip -n red link set lo up
ip -n blue link set lo up

I then try to ping blue from red

# Ping test
ip netns exec red ping 192.168.1.2

But it fails with the error:

connect: Network is unreachable

I then add default gateway for both the namespaces:

# Add default gateway
ip netns exec red ip route add default via 192.168.1.1 dev veth-red
ip netns exec blue ip route add default via 192.168.1.2 dev veth-blue

The ping then works.

My question is, why do I have to add a default gateway if they are on the same network?

Answer 1

I figured this out. Silly mistake. When I added the IP address I did not specify a subnet mask. So it was assuming it to be 255.255.255.255. I changed it to this and it works.

# Set IP Addresses
ip -n red addr add 192.168.1.1/24 dev veth-red
ip -n blue addr add 192.168.1.2/24 dev veth-blue